diff --git a/pkg/analysis_server/lib/lsp_protocol/protocol_custom_generated.dart b/pkg/analysis_server/lib/lsp_protocol/protocol_custom_generated.dart
index fd739c7..046c9c0 100644
--- a/pkg/analysis_server/lib/lsp_protocol/protocol_custom_generated.dart
+++ b/pkg/analysis_server/lib/lsp_protocol/protocol_custom_generated.dart
@@ -188,7 +188,7 @@
   }
 
   final String file;
-  final num offset;
+  final int offset;
 
   Map<String, dynamic> toJson() {
     var __result = <String, dynamic>{};
@@ -226,8 +226,8 @@
           reporter.reportError('must not be null');
           return false;
         }
-        if (!(obj['offset'] is num)) {
-          reporter.reportError('must be of type num');
+        if (!(obj['offset'] is int)) {
+          reporter.reportError('must be of type int');
           return false;
         }
       } finally {
@@ -295,11 +295,11 @@
 
   final String displayUri;
   final String file;
-  final num iLength;
-  final num libId;
-  final num offset;
-  final num rLength;
-  final num rOffset;
+  final int iLength;
+  final int libId;
+  final int offset;
+  final int rLength;
+  final int rOffset;
 
   Map<String, dynamic> toJson() {
     var __result = <String, dynamic>{};
@@ -325,8 +325,8 @@
           reporter.reportError('must not be null');
           return false;
         }
-        if (!(obj['libId'] is num)) {
-          reporter.reportError('must be of type num');
+        if (!(obj['libId'] is int)) {
+          reporter.reportError('must be of type int');
           return false;
         }
       } finally {
@@ -359,8 +359,8 @@
           reporter.reportError('must not be null');
           return false;
         }
-        if (!(obj['rOffset'] is num)) {
-          reporter.reportError('must be of type num');
+        if (!(obj['rOffset'] is int)) {
+          reporter.reportError('must be of type int');
           return false;
         }
       } finally {
@@ -376,8 +376,8 @@
           reporter.reportError('must not be null');
           return false;
         }
-        if (!(obj['iLength'] is num)) {
-          reporter.reportError('must be of type num');
+        if (!(obj['iLength'] is int)) {
+          reporter.reportError('must be of type int');
           return false;
         }
       } finally {
@@ -393,8 +393,8 @@
           reporter.reportError('must not be null');
           return false;
         }
-        if (!(obj['rLength'] is num)) {
-          reporter.reportError('must be of type num');
+        if (!(obj['rLength'] is int)) {
+          reporter.reportError('must be of type int');
           return false;
         }
       } finally {
@@ -427,8 +427,8 @@
           reporter.reportError('must not be null');
           return false;
         }
-        if (!(obj['offset'] is num)) {
-          reporter.reportError('must be of type num');
+        if (!(obj['offset'] is int)) {
+          reporter.reportError('must be of type int');
           return false;
         }
       } finally {
@@ -484,7 +484,7 @@
     return DartDiagnosticServer(port: port);
   }
 
-  final num port;
+  final int port;
 
   Map<String, dynamic> toJson() {
     var __result = <String, dynamic>{};
@@ -504,8 +504,8 @@
           reporter.reportError('must not be null');
           return false;
         }
-        if (!(obj['port'] is num)) {
-          reporter.reportError('must be of type num');
+        if (!(obj['port'] is int)) {
+          reporter.reportError('must be of type int');
           return false;
         }
       } finally {
@@ -1207,7 +1207,7 @@
   }
 
   final String file;
-  final num offset;
+  final int offset;
   final String packageName;
 
   Map<String, dynamic> toJson() {
@@ -1264,8 +1264,8 @@
           reporter.reportError('must not be null');
           return false;
         }
-        if (!(obj['offset'] is num)) {
-          reporter.reportError('must be of type num');
+        if (!(obj['offset'] is int)) {
+          reporter.reportError('must be of type int');
           return false;
         }
       } finally {
diff --git a/pkg/analysis_server/lib/lsp_protocol/protocol_generated.dart b/pkg/analysis_server/lib/lsp_protocol/protocol_generated.dart
index c183159..48503cb 100644
--- a/pkg/analysis_server/lib/lsp_protocol/protocol_generated.dart
+++ b/pkg/analysis_server/lib/lsp_protocol/protocol_generated.dart
@@ -6320,10 +6320,10 @@
   const CompletionItemKind(this._value);
   const CompletionItemKind.fromJson(this._value);
 
-  final num _value;
+  final int _value;
 
   static bool canParse(Object obj, LspJsonReporter reporter) {
-    return obj is num;
+    return obj is int;
   }
 
   static const Text = CompletionItemKind(1);
@@ -6370,10 +6370,10 @@
   const CompletionItemTag(this._value);
   const CompletionItemTag.fromJson(this._value);
 
-  final num _value;
+  final int _value;
 
   static bool canParse(Object obj, LspJsonReporter reporter) {
-    return obj is num;
+    return obj is int;
   }
 
   /// Render a completion as obsolete, usually using a strike-out.
@@ -10715,10 +10715,10 @@
   const DocumentHighlightKind(this._value);
   const DocumentHighlightKind.fromJson(this._value);
 
-  final num _value;
+  final int _value;
 
   static bool canParse(Object obj, LspJsonReporter reporter) {
-    return obj is num;
+    return obj is int;
   }
 
   /// A textual occurrence.
@@ -13758,10 +13758,10 @@
   const FileChangeType(this._value);
   const FileChangeType.fromJson(this._value);
 
-  final num _value;
+  final int _value;
 
   static bool canParse(Object obj, LspJsonReporter reporter) {
-    return obj is num;
+    return obj is int;
   }
 
   /// The file got created.
@@ -16902,7 +16902,7 @@
   const InsertTextFormat._(this._value);
   const InsertTextFormat.fromJson(this._value);
 
-  final num _value;
+  final int _value;
 
   static bool canParse(Object obj, LspJsonReporter reporter) {
     switch (obj) {
@@ -18204,10 +18204,10 @@
   const MessageType(this._value);
   const MessageType.fromJson(this._value);
 
-  final num _value;
+  final int _value;
 
   static bool canParse(Object obj, LspJsonReporter reporter) {
-    return obj is num;
+    return obj is int;
   }
 
   /// An error message.
@@ -19499,10 +19499,10 @@
   ///
   /// If the character value is greater than the line length it defaults back to
   /// the line length.
-  final num character;
+  final int character;
 
   /// Line position in a document (zero-based).
-  final num line;
+  final int line;
 
   Map<String, dynamic> toJson() {
     var __result = <String, dynamic>{};
@@ -19523,8 +19523,8 @@
           reporter.reportError('must not be null');
           return false;
         }
-        if (!(obj['line'] is num)) {
-          reporter.reportError('must be of type num');
+        if (!(obj['line'] is int)) {
+          reporter.reportError('must be of type int');
           return false;
         }
       } finally {
@@ -19540,8 +19540,8 @@
           reporter.reportError('must not be null');
           return false;
         }
-        if (!(obj['character'] is num)) {
-          reporter.reportError('must be of type num');
+        if (!(obj['character'] is int)) {
+          reporter.reportError('must be of type int');
           return false;
         }
       } finally {
@@ -28247,10 +28247,10 @@
   const SymbolKind(this._value);
   const SymbolKind.fromJson(this._value);
 
-  final num _value;
+  final int _value;
 
   static bool canParse(Object obj, LspJsonReporter reporter) {
-    return obj is num;
+    return obj is int;
   }
 
   static const File = SymbolKind(1);
@@ -29926,10 +29926,10 @@
   const TextDocumentSaveReason(this._value);
   const TextDocumentSaveReason.fromJson(this._value);
 
-  final num _value;
+  final int _value;
 
   static bool canParse(Object obj, LspJsonReporter reporter) {
-    return obj is num;
+    return obj is int;
   }
 
   /// Manually triggered, e.g. by the user pressing save, by starting debugging,
@@ -30183,10 +30183,10 @@
   const TextDocumentSyncKind(this._value);
   const TextDocumentSyncKind.fromJson(this._value);
 
-  final num _value;
+  final int _value;
 
   static bool canParse(Object obj, LspJsonReporter reporter) {
-    return obj is num;
+    return obj is int;
   }
 
   /// Documents should not be synced at all.
@@ -31206,10 +31206,10 @@
   const WatchKind(this._value);
   const WatchKind.fromJson(this._value);
 
-  final num _value;
+  final int _value;
 
   static bool canParse(Object obj, LspJsonReporter reporter) {
-    return obj is num;
+    return obj is int;
   }
 
   /// Interested in create events.
diff --git a/pkg/analysis_server/lib/src/analysis_server_abstract.dart b/pkg/analysis_server/lib/src/analysis_server_abstract.dart
index 5fab9db..796e555 100644
--- a/pkg/analysis_server/lib/src/analysis_server_abstract.dart
+++ b/pkg/analysis_server/lib/src/analysis_server_abstract.dart
@@ -73,8 +73,8 @@
   /// The object used to manage the SDK's known to this server.
   final DartSdkManager sdkManager;
 
-  /// The [SearchEngine] for this server, may be `null` if indexing is disabled.
-  SearchEngine? searchEngine;
+  /// The [SearchEngine] for this server.
+  late SearchEngine searchEngine;
 
   late ByteStore byteStore;
 
diff --git a/pkg/analysis_server/lib/src/edit/edit_domain.dart b/pkg/analysis_server/lib/src/edit/edit_domain.dart
index 01eed28..2468770 100644
--- a/pkg/analysis_server/lib/src/edit/edit_domain.dart
+++ b/pkg/analysis_server/lib/src/edit/edit_domain.dart
@@ -85,9 +85,8 @@
   /// [server].
   EditDomainHandler(AnalysisServer server) : super(server) {
     var search = searchEngine = server.searchEngine;
-    refactoringWorkspace = search == null
-        ? null
-        : RefactoringWorkspace(server.driverMap.values, search);
+    refactoringWorkspace =
+        RefactoringWorkspace(server.driverMap.values, search);
     _newRefactoringManager();
   }
 
diff --git a/pkg/analysis_server/lib/src/lsp/channel/lsp_byte_stream_channel.dart b/pkg/analysis_server/lib/src/lsp/channel/lsp_byte_stream_channel.dart
index 993ab3a..2d9aa2c 100644
--- a/pkg/analysis_server/lib/src/lsp/channel/lsp_byte_stream_channel.dart
+++ b/pkg/analysis_server/lib/src/lsp/channel/lsp_byte_stream_channel.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
@@ -51,7 +49,7 @@
 
   @override
   void listen(void Function(Message message) onMessage,
-      {Function onError, void Function() onDone}) {
+      {Function? onError, void Function()? onDone}) {
     _input.transform(LspPacketTransformer()).listen(
       (String data) => _readMessage(data, onMessage),
       onError: onError,
@@ -95,7 +93,7 @@
   }
 
   /// Sends a message prefixed with the required LSP headers.
-  void _sendLsp(Map<String, Object> json) {
+  void _sendLsp(Map<String, dynamic> json) {
     // Don't send any further responses after the communication channel is
     // closed.
     if (_closeRequested) {
diff --git a/pkg/analysis_server/lib/src/lsp/channel/lsp_channel.dart b/pkg/analysis_server/lib/src/lsp/channel/lsp_channel.dart
index 5163e84..8d488b6 100644
--- a/pkg/analysis_server/lib/src/lsp/channel/lsp_channel.dart
+++ b/pkg/analysis_server/lib/src/lsp/channel/lsp_channel.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/src/lsp/lsp_analysis_server.dart';
 
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/commands/organize_imports.dart b/pkg/analysis_server/lib/src/lsp/handlers/commands/organize_imports.dart
index 459a0f3..b57dbbe 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/commands/organize_imports.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/commands/organize_imports.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/constants.dart';
@@ -20,7 +18,7 @@
   String get commandName => 'Organize Imports';
 
   @override
-  Future<ErrorOr<void>> handle(List<dynamic> arguments,
+  Future<ErrorOr<void>> handle(List<dynamic>? arguments,
       ProgressReporter reporter, CancellationToken cancellationToken) async {
     if (arguments == null || arguments.length != 1 || arguments[0] is! String) {
       return ErrorOr.error(ResponseError(
@@ -43,8 +41,8 @@
     }
 
     return result.mapResult((result) {
-      final code = result.content;
-      final unit = result.unit;
+      final code = result.content!;
+      final unit = result.unit!;
 
       if (hasScanParseErrors(result.errors)) {
         // It's not uncommon for editors to run this command automatically on-save
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/commands/perform_refactor.dart b/pkg/analysis_server/lib/src/lsp/handlers/commands/perform_refactor.dart
index fccceba..fff5d0c 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/commands/perform_refactor.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/commands/perform_refactor.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/constants.dart';
@@ -26,7 +24,7 @@
   String get commandName => 'Perform Refactor';
 
   @override
-  Future<ErrorOr<void>> handle(List<dynamic> arguments,
+  Future<ErrorOr<void>> handle(List<dynamic>? arguments,
       ProgressReporter reporter, CancellationToken cancellationToken) async {
     if (arguments == null ||
         arguments.length != 6 ||
@@ -60,26 +58,29 @@
         // If the token we were given is not cancellable, replace it with one that
         // is for the rest of this request, as a future refactor may need to cancel
         // this request.
-        if (cancellationToken is! CancelableToken) {
-          cancellationToken = CancelableToken();
-        }
-        _manager.begin(cancellationToken);
+        // The original token should be kept and also checked for cancellation.
+        final cancelableToken = cancellationToken is CancelableToken
+            ? cancellationToken
+            : CancelableToken();
+        _manager.begin(cancelableToken);
 
         try {
           reporter.begin('Refactoring…');
           final status = await refactoring.checkAllConditions();
 
           if (status.hasError) {
-            return error(ServerErrorCodes.RefactorFailed, status.message);
+            return error(ServerErrorCodes.RefactorFailed, status.message!);
           }
 
-          if (cancellationToken.isCancellationRequested) {
+          if (cancellationToken.isCancellationRequested ||
+              cancelableToken.isCancellationRequested) {
             return error(ErrorCodes.RequestCancelled, 'Request was cancelled');
           }
 
           final change = await refactoring.createChange();
 
-          if (cancellationToken.isCancellationRequested) {
+          if (cancellationToken.isCancellationRequested ||
+              cancelableToken.isCancellationRequested) {
             return error(ErrorCodes.RequestCancelled, 'Request was cancelled');
           }
 
@@ -94,7 +95,7 @@
         } on InconsistentAnalysisException {
           return fileModifiedError;
         } finally {
-          _manager.end(cancellationToken);
+          _manager.end(cancelableToken);
           reporter.end();
         }
       });
@@ -106,7 +107,7 @@
     ResolvedUnitResult result,
     int offset,
     int length,
-    Map<String, dynamic> options,
+    Map<String, dynamic>? options,
   ) async {
     switch (kind) {
       case RefactoringKind.EXTRACT_METHOD:
@@ -168,7 +169,7 @@
 /// Manages a running refactor to help ensure only one refactor runs at a time.
 class _RefactorManager {
   /// The cancellation token for the current in-progress refactor (or null).
-  CancelableToken _currentRefactoringCancellationToken;
+  CancelableToken? _currentRefactoringCancellationToken;
 
   /// Begins a new refactor, cancelling any other in-progress refactors.
   void begin(CancelableToken cancelToken) {
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/commands/send_workspace_edit.dart b/pkg/analysis_server/lib/src/lsp/handlers/commands/send_workspace_edit.dart
index 1f81768..2fbaea1 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/commands/send_workspace_edit.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/commands/send_workspace_edit.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/constants.dart';
@@ -26,7 +24,7 @@
   String get commandName => 'Send Workspace Edit';
 
   @override
-  Future<ErrorOr<void>> handle(List<dynamic> arguments,
+  Future<ErrorOr<void>> handle(List<dynamic>? arguments,
       ProgressReporter reporter, CancellationToken cancellationToken) async {
     if (arguments == null ||
         arguments.length != 1 ||
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/commands/simple_edit_handler.dart b/pkg/analysis_server/lib/src/lsp/handlers/commands/simple_edit_handler.dart
index 5494e3e..df12d35 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/commands/simple_edit_handler.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/commands/simple_edit_handler.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/constants.dart';
@@ -39,9 +37,22 @@
       return success(null);
     }
 
+    final clientCapabilities = server.clientCapabilities;
+    if (clientCapabilities == null) {
+      // This should not happen unless a client misbehaves.
+      return error(ErrorCodes.ServerNotInitialized,
+          'Requests not before server is initilized');
+    }
+
+    final lineInfo = unit.lineInfo;
+    if (lineInfo == null) {
+      return error(ErrorCodes.InternalError,
+          'Unable to produce edits for $docIdentifier as no LineInfo was found');
+    }
+
     final workspaceEdit = toWorkspaceEdit(
-      server.clientCapabilities,
-      [FileEditInformation(docIdentifier, unit.lineInfo, edits)],
+      clientCapabilities,
+      [FileEditInformation(docIdentifier, lineInfo, edits)],
     );
 
     return sendWorkspaceEditToClient(workspaceEdit);
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/commands/sort_members.dart b/pkg/analysis_server/lib/src/lsp/handlers/commands/sort_members.dart
index 5eec574..7fea763 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/commands/sort_members.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/commands/sort_members.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/constants.dart';
@@ -21,7 +19,7 @@
   String get commandName => 'Sort Members';
 
   @override
-  Future<ErrorOr<void>> handle(List<dynamic> arguments,
+  Future<ErrorOr<void>> handle(List<dynamic>? arguments,
       ProgressReporter reporter, CancellationToken cancellationToken) async {
     if (arguments == null || arguments.length != 1 || arguments[0] is! String) {
       return ErrorOr.error(ResponseError(
@@ -38,20 +36,19 @@
     final docIdentifier = server.getVersionedDocumentIdentifier(path);
 
     var driver = server.getAnalysisDriver(path);
-    final result0 = await driver?.parseFile2(path);
+    final result = await driver?.parseFile2(path);
 
     if (cancellationToken.isCancellationRequested) {
       return error(ErrorCodes.RequestCancelled, 'Request was cancelled');
     }
 
-    if (result0 is! ParsedUnitResult) {
+    if (result is! ParsedUnitResult) {
       return ErrorOr.error(ResponseError(
         code: ServerErrorCodes.FileNotAnalyzed,
         message: '$commandName is only available for analyzed files',
       ));
     }
-    // TODO(scheglov) inline after migration
-    var result = result0 as ParsedUnitResult;
+
     final code = result.content;
     final unit = result.unit;
 
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/custom/handler_diagnostic_server.dart b/pkg/analysis_server/lib/src/lsp/handlers/custom/handler_diagnostic_server.dart
index 8c8d812..af7f163 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/custom/handler_diagnostic_server.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/custom/handler_diagnostic_server.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_custom_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
@@ -23,7 +21,13 @@
   @override
   Future<ErrorOr<DartDiagnosticServer>> handle(
       void _, CancellationToken token) async {
-    final port = await server.diagnosticServer.getServerPort();
+    final diagnosticServer = server.diagnosticServer;
+    if (diagnosticServer == null) {
+      return error(ServerErrorCodes.FeatureDisabled,
+          'The diagnostic server is not available');
+    }
+
+    final port = await diagnosticServer.getServerPort();
     return success(DartDiagnosticServer(port: port));
   }
 }
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/custom/handler_reanalyze.dart b/pkg/analysis_server/lib/src/lsp/handlers/custom/handler_reanalyze.dart
index ed8824fc..3b5caac 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/custom/handler_reanalyze.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/custom/handler_reanalyze.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/constants.dart';
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/custom/handler_super.dart b/pkg/analysis_server/lib/src/lsp/handlers/custom/handler_super.dart
index 50f0c5d..af2d137 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/custom/handler_super.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/custom/handler_super.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/constants.dart';
@@ -11,9 +9,10 @@
 import 'package:analysis_server/src/lsp/lsp_analysis_server.dart';
 import 'package:analysis_server/src/lsp/mapping.dart';
 import 'package:analysis_server/src/search/type_hierarchy.dart';
+import 'package:collection/collection.dart';
 
 class SuperHandler
-    extends MessageHandler<TextDocumentPositionParams, Location> {
+    extends MessageHandler<TextDocumentPositionParams, Location?> {
   SuperHandler(LspAnalysisServer server) : super(server);
   @override
   Method get handlesMessage => CustomMethods.super_;
@@ -23,7 +22,7 @@
       TextDocumentPositionParams.jsonHandler;
 
   @override
-  Future<ErrorOr<Location>> handle(
+  Future<ErrorOr<Location?>> handle(
       TextDocumentPositionParams params, CancellationToken token) async {
     if (!isDartDocument(params.textDocument)) {
       return success(null);
@@ -44,8 +43,8 @@
       // finding supers even if the cursor location was inside a method or on its
       // return type.
       var element = server.getElementOfNode(node);
-      while (element == null && node.parent != null) {
-        node = node.parent;
+      while (element == null && node?.parent != null) {
+        node = node?.parent;
         element = server.getElementOfNode(node);
       }
       if (element == null) {
@@ -64,21 +63,22 @@
       // The class will have a memberElement if we were searching for an element
       // otherwise we're looking for a class.
       final isMember = items.first.memberElement != null;
-      final superItem = items.skip(1).firstWhere(
-            (elm) =>
-                isMember ? elm.memberElement != null : elm.classElement != null,
-            orElse: () => null,
-          );
+      final superItem = items.skip(1).firstWhereOrNull(
+          (elm) => isMember ? elm.memberElement != null : true);
 
-      if (superItem == null) {
+      final location = superItem?.memberElement?.location ??
+          superItem?.classElement.location;
+
+      if (location == null) {
         return success(null);
       }
 
-      final location = isMember
-          ? superItem.memberElement.location
-          : superItem.classElement.location;
+      final locationLineInfo = server.getLineInfo(location.file);
+      if (locationLineInfo == null) {
+        return success(null);
+      }
 
-      return success(toLocation(location, server.getLineInfo(location.file)));
+      return success(toLocation(location, locationLineInfo));
     });
   }
 }
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_cancel_request.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_cancel_request.dart
index 46c0e60..0707e09 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_cancel_request.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_cancel_request.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/handlers/handlers.dart';
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_change_workspace_folders.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_change_workspace_folders.dart
index 9b7df1e..9a530d3 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_change_workspace_folders.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_change_workspace_folders.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/handlers/handlers.dart';
@@ -32,13 +30,12 @@
       return success(null);
     }
 
-    final added = params?.event?.added
-        ?.map((wf) => Uri.parse(wf.uri).toFilePath())
-        ?.toList();
+    final added =
+        params.event.added.map((wf) => Uri.parse(wf.uri).toFilePath()).toList();
 
-    final removed = params?.event?.removed
-        ?.map((wf) => Uri.parse(wf.uri).toFilePath())
-        ?.toList();
+    final removed = params.event.removed
+        .map((wf) => Uri.parse(wf.uri).toFilePath())
+        .toList();
 
     server.updateWorkspaceFolders(added, removed);
 
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_code_actions.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_code_actions.dart
index 219138b..c74cdb2 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_code_actions.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_code_actions.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/plugin/edit/assist/assist_core.dart';
@@ -50,18 +48,21 @@
       return success(const []);
     }
 
-    final supportsApplyEdit = server.clientCapabilities.applyEdit;
+    final clientCapabilities = server.clientCapabilities;
+    if (clientCapabilities == null) {
+      // This should not happen unless a client misbehaves.
+      return error(ErrorCodes.ServerNotInitialized,
+          'Requests not before server is initilized');
+    }
 
-    final supportsLiteralCodeActions =
-        server.clientCapabilities.literalCodeActions;
-
-    final supportedKinds = server.clientCapabilities.codeActionKinds;
-
-    final supportedDiagnosticTags = server.clientCapabilities.diagnosticTags;
+    final supportsApplyEdit = clientCapabilities.applyEdit;
+    final supportsLiteralCodeActions = clientCapabilities.literalCodeActions;
+    final supportedKinds = clientCapabilities.codeActionKinds;
+    final supportedDiagnosticTags = clientCapabilities.diagnosticTags;
 
     final unit = await path.mapResult(requireResolvedUnit);
 
-    bool shouldIncludeKind(CodeActionKind kind) {
+    bool shouldIncludeKind(CodeActionKind? kind) {
       /// Checks whether the kind matches the [wanted] kind.
       ///
       /// If `wanted` is `refactor.foo` then:
@@ -72,8 +73,9 @@
           kind == wanted || kind.toString().startsWith('${wanted.toString()}.');
 
       // If the client wants only a specific set, use only that filter.
-      if (params.context?.only != null) {
-        return params.context.only.any(isMatch);
+      final only = params.context.only;
+      if (only != null) {
+        return only.any(isMatch);
       }
 
       // Otherwise, filter out anything not supported by the client (if they
@@ -108,11 +110,14 @@
   }
 
   /// Creates a comparer for [CodeActions] that compares the column distance from [pos].
-  Function(CodeAction a, CodeAction b) _codeActionColumnDistanceComparer(
+  int Function(CodeAction a, CodeAction b) _codeActionColumnDistanceComparer(
       Position pos) {
-    Position posOf(CodeAction action) => action.diagnostics.isNotEmpty
-        ? action.diagnostics.first.range.start
-        : pos;
+    Position posOf(CodeAction action) {
+      final diagnostics = action.diagnostics;
+      return diagnostics != null && diagnostics.isNotEmpty
+          ? diagnostics.first.range.start
+          : pos;
+    }
 
     return (a, b) => _columnDistance(posOf(a), pos)
         .compareTo(_columnDistance(posOf(b), pos));
@@ -174,7 +179,7 @@
   List<CodeAction> _dedupeActions(Iterable<CodeAction> actions, Position pos) {
     final groups = groupBy(actions, (CodeAction action) => action.title);
     return groups.keys.map((title) {
-      final actions = groups[title];
+      final actions = groups[title]!;
 
       // If there's only one in the group, just return it.
       if (actions.length == 1) {
@@ -213,7 +218,7 @@
   }
 
   Future<List<Either2<Command, CodeAction>>> _getAssistActions(
-    bool Function(CodeActionKind) shouldIncludeKind,
+    bool Function(CodeActionKind?) shouldIncludeKind,
     bool supportsLiteralCodeActions,
     Range range,
     int offset,
@@ -248,7 +253,7 @@
   }
 
   Future<ErrorOr<List<Either2<Command, CodeAction>>>> _getCodeActions(
-    bool Function(CodeActionKind) shouldIncludeKind,
+    bool Function(CodeActionKind?) shouldIncludeKind,
     bool supportsLiterals,
     bool supportsWorkspaceApplyEdit,
     Set<DiagnosticTag> supportedDiagnosticTags,
@@ -274,7 +279,7 @@
   }
 
   Future<List<Either2<Command, CodeAction>>> _getFixActions(
-    bool Function(CodeActionKind) shouldIncludeKind,
+    bool Function(CodeActionKind?) shouldIncludeKind,
     bool supportsLiteralCodeActions,
     Set<DiagnosticTag> supportedDiagnosticTags,
     Range range,
@@ -301,10 +306,10 @@
         var workspace = DartChangeWorkspace(server.currentSessions);
         var context = DartFixContextImpl(
             server.instrumentationService, workspace, unit, error, (name) {
-          var tracker = server.declarationsTracker;
+          var tracker = server.declarationsTracker!;
           return TopLevelDeclarationsProvider(tracker).get(
             unit.session.analysisContext,
-            unit.path,
+            unit.path!,
             name,
           );
         });
@@ -357,7 +362,7 @@
       CodeActionKind actionKind,
       String name,
       RefactoringKind refactorKind, [
-      Map<String, dynamic> options,
+      Map<String, dynamic>? options,
     ]) {
       return _commandOrCodeAction(
           supportsLiteralCodeActions,
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_completion.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_completion.dart
index 4f68075..751fdca 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_completion.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_completion.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'dart:math' as math;
 
 import 'package:analysis_server/lsp_protocol/protocol_custom_generated.dart';
@@ -51,15 +49,22 @@
   @override
   Future<ErrorOr<List<CompletionItem>>> handle(
       CompletionParams params, CancellationToken token) async {
+    final clientCapabilities = server.clientCapabilities;
+    if (clientCapabilities == null) {
+      // This should not happen unless a client misbehaves.
+      return error(ErrorCodes.ServerNotInitialized,
+          'Requests not before server is initilized');
+    }
+
     final includeSuggestionSets =
-        suggestFromUnimportedLibraries && server.clientCapabilities.applyEdit;
+        suggestFromUnimportedLibraries && clientCapabilities.applyEdit;
 
     final triggerCharacter = params.context?.triggerCharacter;
     final pos = params.position;
     final path = pathOfDoc(params.textDocument);
     final unit = await path.mapResult(requireResolvedUnit);
 
-    final lineInfo = unit.map<ErrorOr<LineInfo>>(
+    final lineInfo = await unit.map(
       // If we don't have a unit, we can still try to obtain the line info for
       // plugin contributors.
       (error) => path.mapResult(getLineInfo),
@@ -69,13 +74,13 @@
         await lineInfo.mapResult((lineInfo) => toOffset(lineInfo, pos));
 
     return offset.mapResult((offset) async {
-      Future<ErrorOr<List<CompletionItem>>> serverResultsFuture;
+      Future<ErrorOr<List<CompletionItem>>>? serverResultsFuture;
       final pathContext = server.resourceProvider.pathContext;
       final fileExtension = pathContext.extension(path.result);
 
       if (fileExtension == '.dart' && !unit.isError) {
         serverResultsFuture = _getServerDartItems(
-          server.clientCapabilities,
+          clientCapabilities,
           includeSuggestionSets,
           unit.result,
           offset,
@@ -83,7 +88,7 @@
           token,
         );
       } else if (fileExtension == '.yaml') {
-        YamlCompletionGenerator generator;
+        YamlCompletionGenerator? generator;
         if (file_paths.isAnalysisOptionsYaml(pathContext, path.result)) {
           generator = AnalysisOptionsGenerator(server.resourceProvider);
         } else if (file_paths.isFixDataYaml(pathContext, path.result)) {
@@ -95,7 +100,7 @@
         if (generator != null) {
           serverResultsFuture = _getServerYamlItems(
             generator,
-            server.clientCapabilities,
+            clientCapabilities,
             path.result,
             lineInfo.result,
             offset,
@@ -107,7 +112,7 @@
       serverResultsFuture ??= Future.value(success(const <CompletionItem>[]));
 
       final pluginResultsFuture = _getPluginResults(
-          server.clientCapabilities, lineInfo.result, path.result, offset);
+          clientCapabilities, lineInfo.result, path.result, offset);
 
       // Await both server + plugin results together to allow async/IO to
       // overlap.
@@ -138,14 +143,15 @@
       if (importedLibrary == null) continue;
 
       for (var element in import.namespace.definedNames.values) {
-        if (element.librarySource != null) {
-          final declaringLibraryUri = element.librarySource.uri;
-          final elementName = element.name;
+        final librarySource = element.librarySource;
+        final elementName = element.name;
+        if (librarySource != null && elementName != null) {
+          final declaringLibraryUri = librarySource.uri;
 
           final key =
               _createImportedSymbolKey(elementName, declaringLibraryUri);
           alreadyImportedSymbols.putIfAbsent(key, () => <String>{});
-          alreadyImportedSymbols[key]
+          alreadyImportedSymbols[key]!
               .add('${importedLibrary.librarySource.uri}');
         }
       }
@@ -194,12 +200,12 @@
     bool includeSuggestionSets,
     ResolvedUnitResult unit,
     int offset,
-    String triggerCharacter,
+    String? triggerCharacter,
     CancellationToken token,
   ) async {
     final performance = CompletionPerformance();
     performance.path = unit.path;
-    performance.setContentsAndOffset(unit.content, offset);
+    performance.setContentsAndOffset(unit.content!, offset);
     server.performanceStats.completion.add(performance);
 
     return await performance.runRequestOperation((perf) async {
@@ -217,9 +223,9 @@
         }
       }
 
-      Set<ElementKind> includedElementKinds;
-      Set<String> includedElementNames;
-      List<IncludedSuggestionRelevanceTag> includedSuggestionRelevanceTags;
+      Set<ElementKind>? includedElementKinds;
+      Set<String>? includedElementNames;
+      List<IncludedSuggestionRelevanceTag>? includedSuggestionRelevanceTags;
       if (includeSuggestionSets) {
         includedElementKinds = <ElementKind>{};
         includedElementNames = <String>{};
@@ -288,98 +294,101 @@
 
         // Now compute items in suggestion sets.
         var includedSuggestionSets = <IncludedSuggestionSet>[];
-        if (includedElementKinds != null && unit != null) {
+        final declarationsTracker = server.declarationsTracker;
+        if (declarationsTracker != null &&
+            includedElementKinds != null &&
+            includedElementNames != null &&
+            includedSuggestionRelevanceTags != null) {
           computeIncludedSetList(
-            server.declarationsTracker,
+            declarationsTracker,
             unit,
             includedSuggestionSets,
             includedElementNames,
           );
+
+          // Build a fast lookup for imported symbols so that we can filter out
+          // duplicates.
+          final alreadyImportedSymbols = _buildLookupOfImportedSymbols(unit);
+
+          includedSuggestionSets.forEach((includedSet) {
+            final library = declarationsTracker.getLibrary(includedSet.id);
+            if (library == null) {
+              return;
+            }
+
+            // Make a fast lookup for tag relevance.
+            final tagBoosts = <String, int>{};
+            includedSuggestionRelevanceTags!
+                .forEach((t) => tagBoosts[t.tag] = t.relevanceBoost);
+
+            // Only specific types of child declarations should be included.
+            // This list matches what's in _protocolAvailableSuggestion in
+            // the DAS implementation.
+            bool shouldIncludeChild(Declaration child) =>
+                child.kind == DeclarationKind.CONSTRUCTOR ||
+                child.kind == DeclarationKind.ENUM_CONSTANT ||
+                (child.kind == DeclarationKind.GETTER && child.isStatic) ||
+                (child.kind == DeclarationKind.FIELD && child.isStatic);
+
+            // Collect declarations and their children.
+            final allDeclarations = library.declarations
+                .followedBy(library.declarations
+                    .expand((decl) => decl.children.where(shouldIncludeChild)))
+                .toList();
+
+            final setResults = allDeclarations
+                // Filter to only the kinds we should return.
+                .where((item) => includedElementKinds!
+                    .contains(protocolElementKind(item.kind)))
+                .where((item) {
+              // Check existing imports to ensure we don't already import
+              // this element (this exact element from its declaring
+              // library, not just something with the same name). If we do
+              // we'll want to skip it.
+              final declaringUri =
+                  item.parent?.locationLibraryUri ?? item.locationLibraryUri!;
+
+              // For enums and named constructors, only the parent enum/class is in
+              // the list of imported symbols so we use the parents name.
+              final nameKey = item.kind == DeclarationKind.ENUM_CONSTANT ||
+                      item.kind == DeclarationKind.CONSTRUCTOR
+                  ? item.parent!.name
+                  : item.name;
+              final key = _createImportedSymbolKey(nameKey, declaringUri);
+              final importingUris = alreadyImportedSymbols[key];
+
+              // Keep it only if there are either:
+              // - no URIs importing it
+              // - the URIs importing it include this one
+              return importingUris == null ||
+                  importingUris.contains('${library.uri}');
+            }).map((item) => declarationToCompletionItem(
+                      capabilities,
+                      unit.path!,
+                      offset,
+                      includedSet,
+                      library,
+                      tagBoosts,
+                      unit.lineInfo,
+                      item,
+                      completionRequest.replacementOffset,
+                      insertLength,
+                      completionRequest.replacementLength,
+                      // TODO(dantup): Including commit characters in every completion
+                      // increases the payload size. The LSP spec is ambigious
+                      // about how this should be handled (and VS Code requires it) but
+                      // this should be removed (or made conditional based on a capability)
+                      // depending on how the spec is updated.
+                      // https://github.com/microsoft/vscode-languageserver-node/issues/673
+                      includeCommitCharacters:
+                          server.clientConfiguration.previewCommitCharacters,
+                      completeFunctionCalls:
+                          server.clientConfiguration.completeFunctionCalls,
+                    ));
+            results.addAll(setResults);
+          });
         }
 
-        // Build a fast lookup for imported symbols so that we can filter out
-        // duplicates.
-        final alreadyImportedSymbols = _buildLookupOfImportedSymbols(unit);
-
-        includedSuggestionSets.forEach((includedSet) {
-          final library = server.declarationsTracker.getLibrary(includedSet.id);
-          if (library == null) {
-            return;
-          }
-
-          // Make a fast lookup for tag relevance.
-          final tagBoosts = <String, int>{};
-          includedSuggestionRelevanceTags
-              .forEach((t) => tagBoosts[t.tag] = t.relevanceBoost);
-
-          // Only specific types of child declarations should be included.
-          // This list matches what's in _protocolAvailableSuggestion in
-          // the DAS implementation.
-          bool shouldIncludeChild(Declaration child) =>
-              child.kind == DeclarationKind.CONSTRUCTOR ||
-              child.kind == DeclarationKind.ENUM_CONSTANT ||
-              (child.kind == DeclarationKind.GETTER && child.isStatic) ||
-              (child.kind == DeclarationKind.FIELD && child.isStatic);
-
-          // Collect declarations and their children.
-          final allDeclarations = library.declarations
-              .followedBy(library.declarations
-                  .expand((decl) => decl.children.where(shouldIncludeChild)))
-              .toList();
-
-          final setResults = allDeclarations
-              // Filter to only the kinds we should return.
-              .where((item) =>
-                  includedElementKinds.contains(protocolElementKind(item.kind)))
-              .where((item) {
-            // Check existing imports to ensure we don't already import
-            // this element (this exact element from its declaring
-            // library, not just something with the same name). If we do
-            // we'll want to skip it.
-            final declaringUri = item.parent != null
-                ? item.parent.locationLibraryUri
-                : item.locationLibraryUri;
-
-            // For enums and named constructors, only the parent enum/class is in
-            // the list of imported symbols so we use the parents name.
-            final nameKey = item.kind == DeclarationKind.ENUM_CONSTANT ||
-                    item.kind == DeclarationKind.CONSTRUCTOR
-                ? item.parent.name
-                : item.name;
-            final key = _createImportedSymbolKey(nameKey, declaringUri);
-            final importingUris = alreadyImportedSymbols[key];
-
-            // Keep it only if there are either:
-            // - no URIs importing it
-            // - the URIs importing it include this one
-            return importingUris == null ||
-                importingUris.contains('${library.uri}');
-          }).map((item) => declarationToCompletionItem(
-                    capabilities,
-                    unit.path,
-                    offset,
-                    includedSet,
-                    library,
-                    tagBoosts,
-                    unit.lineInfo,
-                    item,
-                    completionRequest.replacementOffset,
-                    insertLength,
-                    completionRequest.replacementLength,
-                    // TODO(dantup): Including commit characters in every completion
-                    // increases the payload size. The LSP spec is ambigious
-                    // about how this should be handled (and VS Code requires it) but
-                    // this should be removed (or made conditional based on a capability)
-                    // depending on how the spec is updated.
-                    // https://github.com/microsoft/vscode-languageserver-node/issues/673
-                    includeCommitCharacters:
-                        server.clientConfiguration.previewCommitCharacters,
-                    completeFunctionCalls:
-                        server.clientConfiguration.completeFunctionCalls,
-                  ));
-          results.addAll(setResults);
-        });
-
         // Perform fuzzy matching based on the identifier in front of the caret to
         // reduce the size of the payload.
         final fuzzyPattern = dartCompletionRequest.targetPrefix;
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_completion_resolve.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_completion_resolve.dart
index d5de4c1..1463107 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_completion_resolve.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_completion_resolve.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_custom_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
@@ -24,7 +22,7 @@
   /// Used to abort previous requests in async handlers if another resolve request
   /// arrives while the previous is being processed (for clients that don't send
   /// cancel events).
-  CompletionItem _latestCompletionItem;
+  CompletionItem? _latestCompletionItem;
 
   CompletionResolveHandler(LspAnalysisServer server) : super(server);
 
@@ -55,6 +53,13 @@
     DartCompletionItemResolutionInfo data,
     CancellationToken token,
   ) async {
+    final clientCapabilities = server.clientCapabilities;
+    if (clientCapabilities == null) {
+      // This should not happen unless a client misbehaves.
+      return error(ErrorCodes.ServerNotInitialized,
+          'Requests not before server is initilized');
+    }
+
     final lineInfo = server.getLineInfo(data.file);
     if (lineInfo == null) {
       return error(
@@ -68,7 +73,7 @@
     // extracting (with support for the different types of responses between
     // the servers). Where is an appropriate place to put it?
 
-    var library = server.declarationsTracker.getLibrary(data.libId);
+    var library = server.declarationsTracker?.getLibrary(data.libId);
     if (library == null) {
       return error(
         ErrorCodes.InvalidParams,
@@ -94,16 +99,19 @@
     _latestCompletionItem = item;
     while (item == _latestCompletionItem && timer.elapsed < timeout) {
       try {
-        var analysisDriver = server.getAnalysisDriver(data.file);
-        var session = analysisDriver.currentSession;
+        final analysisDriver = server.getAnalysisDriver(data.file);
+        final session = analysisDriver?.currentSession;
 
-        if (token.isCancellationRequested) {
+        // We shouldn't not get a driver/session, but if we did perhaps the file
+        // was removed from the analysis set so assume the request is no longer
+        // valid.
+        if (session == null || token.isCancellationRequested) {
           return cancelled();
         }
 
         analyzer.LibraryElement requestedLibraryElement;
         {
-          var result = await session.getLibraryByUri2(library.uriStr);
+          final result = await session.getLibraryByUri2(library.uriStr);
           if (result is LibraryElementResult) {
             requestedLibraryElement = result.element;
           } else {
@@ -150,7 +158,7 @@
 
         // If this completion involves editing other files, we'll need to build
         // a command that the client will call to apply those edits later.
-        Command command;
+        Command? command;
         if (otherFilesChanges.isNotEmpty) {
           final workspaceEdit =
               createPlainWorkspaceEdit(server, otherFilesChanges);
@@ -161,19 +169,19 @@
         }
 
         // Documentation is added on during resolve for LSP.
-        final formats =
-            server.clientCapabilities.completionDocumentationFormats;
+        final formats = clientCapabilities.completionDocumentationFormats;
         final supportsInsertReplace =
-            server.clientCapabilities.insertReplaceCompletionRanges;
+            clientCapabilities.insertReplaceCompletionRanges;
         final dartDoc =
             analyzer.getDartDocPlainText(requestedElement.documentationComment);
-        final documentation = asStringOrMarkupContent(formats, dartDoc);
+        final documentation =
+            dartDoc != null ? asStringOrMarkupContent(formats, dartDoc) : null;
 
         return success(CompletionItem(
           label: item.label,
           kind: item.kind,
           tags: item.tags,
-          detail: data.displayUri != null && thisFilesChanges.isNotEmpty
+          detail: thisFilesChanges.isNotEmpty
               ? "Auto import from '${data.displayUri}'\n\n${item.detail ?? ''}"
                   .trim()
               : item.detail,
@@ -234,13 +242,14 @@
       return cancelled();
     }
 
+    final description = packageDetails?.description;
     return success(CompletionItem(
       label: item.label,
       kind: item.kind,
       tags: item.tags,
       detail: item.detail,
-      documentation: packageDetails?.description != null
-          ? Either2<String, MarkupContent>.t1(packageDetails.description)
+      documentation: description != null
+          ? Either2<String, MarkupContent>.t1(description)
           : null,
       deprecated: item.deprecated,
       preselect: item.preselect,
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_definition.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_definition.dart
index 5b52c33..a9bc050 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_definition.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_definition.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/protocol/protocol_generated.dart'
@@ -18,6 +16,7 @@
 import 'package:analyzer_plugin/protocol/protocol_generated.dart' as plugin;
 import 'package:analyzer_plugin/src/utilities/navigation/navigation.dart';
 import 'package:analyzer_plugin/utilities/navigation/navigation_dart.dart';
+import 'package:collection/collection.dart';
 
 class DefinitionHandler extends MessageHandler<TextDocumentPositionParams,
         Either2<List<Location>, List<LocationLink>>>
@@ -53,9 +52,10 @@
         NavigationCollectorImpl(collectCodeLocations: supportsLocationLink);
 
     final result = await server.getResolvedUnit(path);
-    if (result?.state == ResultState.VALID) {
+    final unit = result?.unit;
+    if (result?.state == ResultState.VALID && unit != null) {
       computeDartNavigation(
-          server.resourceProvider, collector, result.unit, offset, 0);
+          server.resourceProvider, collector, unit, offset, 0);
       collector.createRegions();
     }
 
@@ -66,8 +66,14 @@
   @override
   Future<ErrorOr<Either2<List<Location>, List<LocationLink>>>> handle(
       TextDocumentPositionParams params, CancellationToken token) async {
-    final supportsLocationLink =
-        server.clientCapabilities.definitionLocationLink;
+    final clientCapabilities = server.clientCapabilities;
+    if (clientCapabilities == null) {
+      // This should not happen unless a client misbehaves.
+      return error(ErrorCodes.ServerNotInitialized,
+          'Requests not before server is initilized');
+    }
+
+    final supportsLocationLink = clientCapabilities.definitionLocationLink;
 
     final pos = params.position;
     final path = pathOfDoc(params.textDocument);
@@ -94,20 +100,27 @@
         final mergedResults = merger.mergeNavigation(allResults);
         final mergedTargets = mergedResults?.targets ?? [];
 
+        if (mergedResults == null) {
+          return success(
+            Either2<List<Location>, List<LocationLink>>.t1(const []),
+          );
+        }
+
         // Convert and filter the results using the correct type of Location class
         // depending on the client capabilities.
         if (supportsLocationLink) {
           final convertedResults = convert(
             mergedTargets,
-            (target) => _toLocationLink(mergedResults, lineInfo, target),
-          ).toList();
+            (NavigationTarget target) =>
+                _toLocationLink(mergedResults, lineInfo, target),
+          ).whereNotNull().toList();
 
           final results = _filterResults(
             convertedResults,
             params.textDocument.uri,
             pos.line,
-            (element) => element.targetUri,
-            (element) => element.targetSelectionRange,
+            (LocationLink element) => element.targetUri,
+            (LocationLink element) => element.targetSelectionRange,
           );
 
           return success(
@@ -116,15 +129,15 @@
         } else {
           final convertedResults = convert(
             mergedTargets,
-            (target) => _toLocation(mergedResults, target),
-          ).toList();
+            (NavigationTarget target) => _toLocation(mergedResults, target),
+          ).whereNotNull().toList();
 
           final results = _filterResults(
             convertedResults,
             params.textDocument.uri,
             pos.line,
-            (element) => element.uri,
-            (element) => element.range,
+            (Location element) => element.uri,
+            (Location element) => element.range,
           );
 
           return success(
@@ -159,19 +172,24 @@
     return otherResults.isNotEmpty ? otherResults : results;
   }
 
-  Location _toLocation(
+  Location? _toLocation(
       AnalysisNavigationParams mergedResults, NavigationTarget target) {
     final targetFilePath = mergedResults.files[target.fileIndex];
     final targetLineInfo = server.getLineInfo(targetFilePath);
-    return navigationTargetToLocation(targetFilePath, target, targetLineInfo);
+    return targetLineInfo != null
+        ? navigationTargetToLocation(targetFilePath, target, targetLineInfo)
+        : null;
   }
 
-  LocationLink _toLocationLink(AnalysisNavigationParams mergedResults,
+  LocationLink? _toLocationLink(AnalysisNavigationParams mergedResults,
       LineInfo sourceLineInfo, NavigationTarget target) {
     final region = mergedResults.regions.first;
     final targetFilePath = mergedResults.files[target.fileIndex];
     final targetLineInfo = server.getLineInfo(targetFilePath);
-    return navigationTargetToLocationLink(
-        region, sourceLineInfo, targetFilePath, target, targetLineInfo);
+
+    return targetLineInfo != null
+        ? navigationTargetToLocationLink(
+            region, sourceLineInfo, targetFilePath, target, targetLineInfo)
+        : null;
   }
 }
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_document_highlights.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_document_highlights.dart
index 9573acc..ff213a4 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_document_highlights.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_document_highlights.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/domains/analysis/occurrences.dart';
@@ -13,7 +11,7 @@
 import 'package:analysis_server/src/lsp/mapping.dart';
 
 class DocumentHighlightsHandler extends MessageHandler<
-    TextDocumentPositionParams, List<DocumentHighlight>> {
+    TextDocumentPositionParams, List<DocumentHighlight>?> {
   DocumentHighlightsHandler(LspAnalysisServer server) : super(server);
   @override
   Method get handlesMessage => Method.textDocument_documentHighlight;
@@ -23,7 +21,7 @@
       TextDocumentPositionParams.jsonHandler;
 
   @override
-  Future<ErrorOr<List<DocumentHighlight>>> handle(
+  Future<ErrorOr<List<DocumentHighlight>?>> handle(
       TextDocumentPositionParams params, CancellationToken token) async {
     if (!isDartDocument(params.textDocument)) {
       return success(const []);
@@ -36,7 +34,7 @@
 
     return offset.mapResult((requestedOffset) {
       final collector = OccurrencesCollectorImpl();
-      addDartOccurrences(collector, unit.result.unit);
+      addDartOccurrences(collector, unit.result.unit!);
 
       // Find an occurrence that has an instance that spans the position.
       for (final occurrence in collector.allOccurrences) {
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_document_symbols.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_document_symbols.dart
index b4b6f14..0c1141d 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_document_symbols.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_document_symbols.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/computer/computer_outline.dart';
@@ -16,7 +14,7 @@
 import 'package:analyzer/source/line_info.dart';
 
 class DocumentSymbolHandler extends MessageHandler<DocumentSymbolParams,
-    Either2<List<DocumentSymbol>, List<SymbolInformation>>> {
+    Either2<List<DocumentSymbol>, List<SymbolInformation>>?> {
   DocumentSymbolHandler(LspAnalysisServer server) : super(server);
   @override
   Method get handlesMessage => Method.textDocument_documentSymbol;
@@ -26,9 +24,10 @@
       DocumentSymbolParams.jsonHandler;
 
   @override
-  Future<ErrorOr<Either2<List<DocumentSymbol>, List<SymbolInformation>>>>
+  Future<ErrorOr<Either2<List<DocumentSymbol>, List<SymbolInformation>>?>>
       handle(DocumentSymbolParams params, CancellationToken token) async {
-    if (!isDartDocument(params.textDocument)) {
+    final clientCapabilities = server.clientCapabilities;
+    if (clientCapabilities == null || !isDartDocument(params.textDocument)) {
       return success(
         Either2<List<DocumentSymbol>, List<SymbolInformation>>.t2([]),
       );
@@ -37,7 +36,7 @@
     final path = pathOfDoc(params.textDocument);
     final unit = await path.mapResult(requireResolvedUnit);
     return unit.mapResult(
-        (unit) => _getSymbols(server.clientCapabilities, path.result, unit));
+        (unit) => _getSymbols(clientCapabilities, path.result, unit));
   }
 
   DocumentSymbol _asDocumentSymbol(
@@ -45,41 +44,49 @@
     LineInfo lineInfo,
     Outline outline,
   ) {
+    final codeRange = toRange(lineInfo, outline.codeOffset, outline.codeLength);
+    final nameLocation = outline.element.location;
+    final nameRange = nameLocation != null
+        ? toRange(lineInfo, nameLocation.offset, nameLocation.length)
+        : null;
     return DocumentSymbol(
       name: toElementName(outline.element),
       detail: outline.element.parameters,
       kind: elementKindToSymbolKind(supportedKinds, outline.element.kind),
       deprecated: outline.element.isDeprecated,
-      range: toRange(lineInfo, outline.codeOffset, outline.codeLength),
-      selectionRange: toRange(lineInfo, outline.element.location.offset,
-          outline.element.location.length),
+      range: codeRange,
+      selectionRange: nameRange ?? codeRange,
       children: outline.children
           ?.map((child) => _asDocumentSymbol(supportedKinds, lineInfo, child))
-          ?.toList(),
+          .toList(),
     );
   }
 
-  SymbolInformation _asSymbolInformation(
-    String containerName,
+  SymbolInformation? _asSymbolInformation(
+    String? containerName,
     Set<SymbolKind> supportedKinds,
     String documentUri,
     LineInfo lineInfo,
     Outline outline,
   ) {
+    final location = outline.element.location;
+    if (location == null) {
+      return null;
+    }
+
     return SymbolInformation(
       name: toElementName(outline.element),
       kind: elementKindToSymbolKind(supportedKinds, outline.element.kind),
       deprecated: outline.element.isDeprecated,
       location: Location(
         uri: documentUri,
-        range: toRange(lineInfo, outline.element.location.offset,
-            outline.element.location.length),
+        range: toRange(lineInfo, location.offset, location.length),
       ),
       containerName: containerName,
     );
   }
 
-  ErrorOr<Either2<List<DocumentSymbol>, List<SymbolInformation>>> _getSymbols(
+  ErrorOr<Either2<List<DocumentSymbol>, List<SymbolInformation>>?> _getSymbols(
     LspClientCapabilities capabilities,
     String path,
     ResolvedUnitResult unit,
@@ -90,12 +97,16 @@
     if (capabilities.hierarchicalSymbols) {
       // Return a tree of DocumentSymbol only if the client shows explicit support
       // for it.
+      final children = outline.children;
+      if (children == null) {
+        return success(null);
+      }
       return success(
         Either2<List<DocumentSymbol>, List<SymbolInformation>>.t1(
-          outline?.children
-              ?.map((child) => _asDocumentSymbol(
+          children
+              .map((child) => _asDocumentSymbol(
                   capabilities.documentSymbolKinds, unit.lineInfo, child))
-              ?.toList(),
+              .toList(),
         ),
       );
     } else {
@@ -105,20 +116,23 @@
 
       // Adds a symbol and it's children recursively, supplying the parent
       // name as required by SymbolInformation.
-      void addSymbol(Outline outline, {String parentName}) {
-        allSymbols.add(_asSymbolInformation(
+      void addSymbol(Outline outline, {String? parentName}) {
+        final symbol = _asSymbolInformation(
           parentName,
           capabilities.documentSymbolKinds,
           documentUri,
           unit.lineInfo,
           outline,
-        ));
+        );
+        if (symbol != null) {
+          allSymbols.add(symbol);
+        }
         outline.children?.forEach(
           (c) => addSymbol(c, parentName: outline.element.name),
         );
       }
 
-      outline?.children?.forEach(addSymbol);
+      outline.children?.forEach(addSymbol);
 
       return success(
         Either2<List<DocumentSymbol>, List<SymbolInformation>>.t2(allSymbols),
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_execute_command.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_execute_command.dart
index 40e0e03..533798f 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_execute_command.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_execute_command.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/constants.dart';
@@ -18,7 +16,7 @@
 /// Handles workspace/executeCommand messages by delegating to a specific handler
 /// based on the command.
 class ExecuteCommandHandler
-    extends MessageHandler<ExecuteCommandParams, Object> {
+    extends MessageHandler<ExecuteCommandParams, Object?> {
   final Map<String, CommandHandler> commandHandlers;
   ExecuteCommandHandler(LspAnalysisServer server)
       : commandHandlers = {
@@ -37,7 +35,7 @@
       ExecuteCommandParams.jsonHandler;
 
   @override
-  Future<ErrorOr<Object>> handle(
+  Future<ErrorOr<Object?>> handle(
       ExecuteCommandParams params, CancellationToken cancellationToken) async {
     final handler = commandHandlers[params.command];
     if (handler == null) {
@@ -45,9 +43,10 @@
           '${params.command} is not a valid command identifier', null);
     }
 
-    final progress = params.workDoneToken != null
-        ? ProgressReporter.clientProvided(server, params.workDoneToken)
-        : server.clientCapabilities.workDoneProgress
+    final workDoneToken = params.workDoneToken;
+    final progress = workDoneToken != null
+        ? ProgressReporter.clientProvided(server, workDoneToken)
+        : server.clientCapabilities?.workDoneProgress ?? false
             ? ProgressReporter.serverCreated(server)
             : ProgressReporter.noop;
     return handler.handle(params.arguments, progress, cancellationToken);
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_exit.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_exit.dart
index 89a4776..a5850a5 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_exit.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_exit.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'dart:io';
 
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_folding.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_folding.dart
index cda2b35..d67f740 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_folding.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_folding.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/computer/computer_folding.dart';
@@ -31,11 +29,11 @@
 
     return path.mapResult((path) async {
       final partialResults = <List<FoldingRegion>>[];
-      LineInfo lineInfo;
+      LineInfo? lineInfo;
 
       final unit = server.getParsedUnit(path);
       if (unit?.state == ResultState.VALID) {
-        lineInfo = unit.lineInfo;
+        lineInfo = unit!.lineInfo;
 
         final regions = DartUnitFoldingComputer(lineInfo, unit.unit).compute();
         partialResults.insert(0, regions);
@@ -59,7 +57,7 @@
           notificationManager.merger.mergeFoldingRegions(partialResults);
 
       return success(
-        regions.map((region) => toFoldingRange(lineInfo, region)).toList(),
+        regions.map((region) => toFoldingRange(lineInfo!, region)).toList(),
       );
     });
   }
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_format_on_type.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_format_on_type.dart
index 2c3b193..1ed3a71 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_format_on_type.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_format_on_type.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/constants.dart';
@@ -14,7 +12,7 @@
 import 'package:analyzer/dart/analysis/results.dart';
 
 class FormatOnTypeHandler
-    extends MessageHandler<DocumentOnTypeFormattingParams, List<TextEdit>> {
+    extends MessageHandler<DocumentOnTypeFormattingParams, List<TextEdit>?> {
   FormatOnTypeHandler(LspAnalysisServer server) : super(server);
   @override
   Method get handlesMessage => Method.textDocument_onTypeFormatting;
@@ -23,14 +21,14 @@
   LspJsonHandler<DocumentOnTypeFormattingParams> get jsonHandler =>
       DocumentOnTypeFormattingParams.jsonHandler;
 
-  ErrorOr<List<TextEdit>> formatFile(String path) {
+  ErrorOr<List<TextEdit>?> formatFile(String path) {
     final file = server.resourceProvider.getFile(path);
     if (!file.exists) {
       return error(ServerErrorCodes.InvalidFilePath, 'Invalid file path', path);
     }
 
     final result = server.getParsedUnit(path);
-    if (result.state != ResultState.VALID || result.errors.isNotEmpty) {
+    if (result?.state != ResultState.VALID || result!.errors.isNotEmpty) {
       return success(null);
     }
 
@@ -39,7 +37,7 @@
   }
 
   @override
-  Future<ErrorOr<List<TextEdit>>> handle(
+  Future<ErrorOr<List<TextEdit>?>> handle(
       DocumentOnTypeFormattingParams params, CancellationToken token) async {
     if (!isDartDocument(params.textDocument)) {
       return success(null);
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_format_range.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_format_range.dart
index 934cd2a..d6f7247 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_format_range.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_format_range.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/constants.dart';
@@ -14,7 +12,7 @@
 import 'package:analyzer/dart/analysis/results.dart';
 
 class FormatRangeHandler
-    extends MessageHandler<DocumentRangeFormattingParams, List<TextEdit>> {
+    extends MessageHandler<DocumentRangeFormattingParams, List<TextEdit>?> {
   FormatRangeHandler(LspAnalysisServer server) : super(server);
   @override
   Method get handlesMessage => Method.textDocument_rangeFormatting;
@@ -23,14 +21,14 @@
   LspJsonHandler<DocumentRangeFormattingParams> get jsonHandler =>
       DocumentRangeFormattingParams.jsonHandler;
 
-  ErrorOr<List<TextEdit>> formatRange(String path, Range range) {
+  ErrorOr<List<TextEdit>?> formatRange(String path, Range range) {
     final file = server.resourceProvider.getFile(path);
     if (!file.exists) {
       return error(ServerErrorCodes.InvalidFilePath, 'Invalid file path', path);
     }
 
     final result = server.getParsedUnit(path);
-    if (result.state != ResultState.VALID || result.errors.isNotEmpty) {
+    if (result?.state != ResultState.VALID || result!.errors.isNotEmpty) {
       return success(null);
     }
 
@@ -40,7 +38,7 @@
   }
 
   @override
-  Future<ErrorOr<List<TextEdit>>> handle(
+  Future<ErrorOr<List<TextEdit>?>> handle(
       DocumentRangeFormattingParams params, CancellationToken token) async {
     if (!isDartDocument(params.textDocument)) {
       return success(null);
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_formatting.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_formatting.dart
index e2cf8e1..75b7a41 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_formatting.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_formatting.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/constants.dart';
@@ -14,7 +12,7 @@
 import 'package:analyzer/dart/analysis/results.dart';
 
 class FormattingHandler
-    extends MessageHandler<DocumentFormattingParams, List<TextEdit>> {
+    extends MessageHandler<DocumentFormattingParams, List<TextEdit>?> {
   FormattingHandler(LspAnalysisServer server) : super(server);
   @override
   Method get handlesMessage => Method.textDocument_formatting;
@@ -23,14 +21,14 @@
   LspJsonHandler<DocumentFormattingParams> get jsonHandler =>
       DocumentFormattingParams.jsonHandler;
 
-  ErrorOr<List<TextEdit>> formatFile(String path) {
+  ErrorOr<List<TextEdit>?> formatFile(String path) {
     final file = server.resourceProvider.getFile(path);
     if (!file.exists) {
       return error(ServerErrorCodes.InvalidFilePath, 'Invalid file path', path);
     }
 
     final result = server.getParsedUnit(path);
-    if (result.state != ResultState.VALID || result.errors.isNotEmpty) {
+    if (result?.state != ResultState.VALID || result!.errors.isNotEmpty) {
       return success(null);
     }
 
@@ -39,7 +37,7 @@
   }
 
   @override
-  Future<ErrorOr<List<TextEdit>>> handle(
+  Future<ErrorOr<List<TextEdit>?>> handle(
       DocumentFormattingParams params, CancellationToken token) async {
     if (!isDartDocument(params.textDocument)) {
       return success(null);
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_hover.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_hover.dart
index 8c82497..1bef994 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_hover.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_hover.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/protocol/protocol_generated.dart';
@@ -15,7 +13,7 @@
 import 'package:analyzer/dart/analysis/results.dart';
 import 'package:analyzer/source/line_info.dart';
 
-class HoverHandler extends MessageHandler<TextDocumentPositionParams, Hover> {
+class HoverHandler extends MessageHandler<TextDocumentPositionParams, Hover?> {
   HoverHandler(LspAnalysisServer server) : super(server);
   @override
   Method get handlesMessage => Method.textDocument_hover;
@@ -25,7 +23,7 @@
       TextDocumentPositionParams.jsonHandler;
 
   @override
-  Future<ErrorOr<Hover>> handle(
+  Future<ErrorOr<Hover?>> handle(
       TextDocumentPositionParams params, CancellationToken token) async {
     if (!isDartDocument(params.textDocument)) {
       return success(null);
@@ -38,7 +36,7 @@
     return offset.mapResult((offset) => _getHover(unit.result, offset));
   }
 
-  Hover toHover(LineInfo lineInfo, HoverInformation hover) {
+  Hover? toHover(LineInfo lineInfo, HoverInformation? hover) {
     if (hover == null) {
       return null;
     }
@@ -55,16 +53,16 @@
     // Description.
     if (hover.elementDescription != null) {
       content.writeln('```dart');
-      if (hover.isDeprecated) {
+      if (hover.isDeprecated ?? false) {
         content.write('(deprecated) ');
       }
       content..writeln(hover.elementDescription)..writeln('```');
     }
 
     // Source library.
-    if (hover.containingLibraryName != null &&
-        hover.containingLibraryName.isNotEmpty) {
-      content..writeln('*${hover.containingLibraryName}*')..writeln();
+    final containingLibraryName = hover.containingLibraryName;
+    if (containingLibraryName != null && containingLibraryName.isNotEmpty) {
+      content..writeln('*$containingLibraryName*')..writeln();
     }
 
     // Doc comments.
@@ -75,7 +73,7 @@
       content.writeln(cleanDartdoc(hover.dartdoc));
     }
 
-    final formats = server.clientCapabilities.hoverContentFormats;
+    final formats = server.clientCapabilities?.hoverContentFormats;
     return Hover(
       contents:
           asStringOrMarkupContent(formats, content.toString().trimRight()),
@@ -83,9 +81,14 @@
     );
   }
 
-  ErrorOr<Hover> _getHover(ResolvedUnitResult unit, int offset) {
+  ErrorOr<Hover?> _getHover(ResolvedUnitResult unit, int offset) {
+    final compilationUnit = unit.unit;
+    if (compilationUnit == null) {
+      return success(null);
+    }
+
     final hover = DartUnitHoverComputer(
-            server.getDartdocDirectiveInfoFor(unit), unit.unit, offset)
+            server.getDartdocDirectiveInfoFor(unit), compilationUnit, offset)
         .compute();
     return success(toHover(unit.lineInfo, hover));
   }
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_implementation.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_implementation.dart
index 822398c..ca6c915 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_implementation.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_implementation.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/protocol/protocol_generated.dart';
@@ -11,6 +9,7 @@
 import 'package:analysis_server/src/lsp/lsp_analysis_server.dart';
 import 'package:analysis_server/src/lsp/mapping.dart';
 import 'package:analysis_server/src/search/type_hierarchy.dart';
+import 'package:collection/collection.dart';
 
 class ImplementationHandler
     extends MessageHandler<TextDocumentPositionParams, List<Location>> {
@@ -38,22 +37,27 @@
   }
 
   Future<ErrorOr<List<Location>>> _getImplementations(
-      String file, int offset, CancelableToken token) async {
+      String file, int offset, CancellationToken token) async {
     final element = await server.getElementAtOffset(file, offset);
+    if (element == null) {
+      return success([]);
+    }
+
     final computer = TypeHierarchyComputer(server.searchEngine, element);
+
     if (token.isCancellationRequested) {
       return cancelled();
     }
-    final items = await computer.compute();
 
+    final items = await computer.compute();
     if (items == null || items.isEmpty) {
       return success([]);
     }
 
     Iterable<TypeHierarchyItem> getDescendants(TypeHierarchyItem item) => item
         .subclasses
-        ?.map((i) => items[i])
-        ?.followedBy(item.subclasses.expand((i) => getDescendants(items[i])));
+        .map((i) => items[i])
+        .followedBy(item.subclasses.expand((i) => getDescendants(items[i])));
 
     // [TypeHierarchyComputer] returns the whole tree, but we specifically only
     // want implementations (sub-classes). Find the referenced element and then
@@ -61,8 +65,9 @@
     var currentItem = items.firstWhere(
       (item) {
         final location =
-            item.memberElement?.location ?? item.classElement?.location;
-        return location.offset <= offset &&
+            item.memberElement?.location ?? item.classElement.location;
+        return location != null &&
+            location.offset <= offset &&
             location.offset + location.length >= offset;
       },
       // If we didn't find an item spanning our offset, we must've been at a
@@ -70,23 +75,32 @@
       orElse: () => items.first,
     );
 
-    final isClass = currentItem.memberElement == null;
+    final isMember = currentItem.memberElement != null;
+    final locations = getDescendants(currentItem)
+        // Filter based on type, so when searching for members we don't include
+        // any intermediate classes that don't have implementations for the
+        // method.
+        .where((item) => isMember ? item.memberElement != null : true)
+        .map((item) {
+          final elementLocation =
+              item.memberElement?.location ?? item.classElement.location;
+          if (elementLocation == null) {
+            return null;
+          }
 
-    final locations = getDescendants(currentItem).where((item) {
-      // Filter based on type, so when searching for members we don't include
-      // any intermediate classes that don't have implementations for the
-      // method.
-      return isClass ? item.classElement != null : item.memberElement != null;
-    }).map((item) {
-      final elementLocation =
-          item.memberElement?.location ?? item.classElement?.location;
-      final lineInfo = server.getLineInfo(elementLocation.file);
-      return Location(
-        uri: Uri.file(elementLocation.file).toString(),
-        range:
-            toRange(lineInfo, elementLocation.offset, elementLocation.length),
-      );
-    }).toList();
+          final lineInfo = server.getLineInfo(elementLocation.file);
+          if (lineInfo == null) {
+            return null;
+          }
+
+          return Location(
+            uri: Uri.file(elementLocation.file).toString(),
+            range: toRange(
+                lineInfo, elementLocation.offset, elementLocation.length),
+          );
+        })
+        .whereNotNull()
+        .toList();
 
     return success(locations);
   }
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_initialize.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_initialize.dart
index 091f1e0..146b2c1 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_initialize.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_initialize.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'dart:io';
 
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
@@ -32,12 +30,15 @@
     );
 
     final openWorkspacePaths = <String>[];
+    final workspaceFolders = params.workspaceFolders;
+    final rootUri = params.rootUri;
+    final rootPath = params.rootPath;
     // The onlyAnalyzeProjectsWithOpenFiles flag allows opening huge folders
     // without setting them as analysis roots. Instead, analysis roots will be
     // based only on the open files.
     if (!server.initializationOptions.onlyAnalyzeProjectsWithOpenFiles) {
-      if (params.workspaceFolders != null) {
-        params.workspaceFolders.forEach((wf) {
+      if (workspaceFolders != null) {
+        workspaceFolders.forEach((wf) {
           final uri = Uri.parse(wf.uri);
           // Only file URIs are supported, but there's no way to signal this to
           // the LSP client (and certainly not before initialization).
@@ -46,13 +47,13 @@
           }
         });
       }
-      if (params.rootUri != null) {
-        final uri = Uri.parse(params.rootUri);
+      if (rootUri != null) {
+        final uri = Uri.parse(rootUri);
         if (uri.isScheme('file')) {
           openWorkspacePaths.add(uri.toFilePath());
         }
-      } else if (params.rootPath != null) {
-        openWorkspacePaths.add(params.rootPath);
+      } else if (rootPath != null) {
+        openWorkspacePaths.add(rootPath);
       }
     }
 
@@ -61,8 +62,9 @@
       openWorkspacePaths,
     );
 
-    server.capabilities = server.capabilitiesComputer
-        .computeServerCapabilities(server.clientCapabilities);
+    final capabilities = server.capabilitiesComputer
+        .computeServerCapabilities(server.clientCapabilities!);
+    server.capabilities = capabilities;
 
     var sdkVersion = Platform.version;
     if (sdkVersion.contains(' ')) {
@@ -70,7 +72,7 @@
     }
 
     return success(InitializeResult(
-      capabilities: server.capabilities,
+      capabilities: capabilities,
       serverInfo: InitializeResultServerInfo(
         name: 'Dart SDK LSP Analysis Server',
         version: sdkVersion,
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_initialized.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_initialized.dart
index 114216b..d842bbb 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_initialized.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_initialized.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/handlers/handler_states.dart';
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_references.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_references.dart
index bfd1405..b302227 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_references.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_references.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/handlers/handlers.dart';
@@ -17,9 +15,10 @@
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer_plugin/src/utilities/navigation/navigation.dart';
 import 'package:analyzer_plugin/utilities/navigation/navigation_dart.dart';
+import 'package:collection/collection.dart';
 
 class ReferencesHandler
-    extends MessageHandler<ReferenceParams, List<Location>> {
+    extends MessageHandler<ReferenceParams, List<Location>?> {
   ReferencesHandler(LspAnalysisServer server) : super(server);
   @override
   Method get handlesMessage => Method.textDocument_references;
@@ -29,7 +28,7 @@
       ReferenceParams.jsonHandler;
 
   @override
-  Future<ErrorOr<List<Location>>> handle(
+  Future<ErrorOr<List<Location>?>> handle(
       ReferenceParams params, CancellationToken token) async {
     if (!isDartDocument(params.textDocument)) {
       return success(const []);
@@ -50,21 +49,23 @@
     return convert(collector.targets, (NavigationTarget target) {
       final targetFilePath = collector.files[target.fileIndex];
       final lineInfo = server.getLineInfo(targetFilePath);
-      return navigationTargetToLocation(targetFilePath, target, lineInfo);
-    }).toList();
+      return lineInfo != null
+          ? navigationTargetToLocation(targetFilePath, target, lineInfo)
+          : null;
+    }).whereNotNull().toList();
   }
 
-  Future<ErrorOr<List<Location>>> _getRefererences(String path, int offset,
+  Future<ErrorOr<List<Location>?>> _getRefererences(String path, int offset,
       ReferenceParams params, ResolvedUnitResult unit) async {
     var element = await server.getElementAtOffset(path, offset);
     if (element is ImportElement) {
-      element = (element as ImportElement).prefix;
+      element = element.prefix;
     }
     if (element is FieldFormalParameterElement) {
-      element = (element as FieldFormalParameterElement).field;
+      element = element.field;
     }
     if (element is PropertyAccessorElement) {
-      element = (element as PropertyAccessorElement).variable;
+      element = element.variable;
     }
     if (element == null) {
       return success(null);
@@ -73,16 +74,18 @@
     final computer = ElementReferencesComputer(server.searchEngine);
     final results = await computer.compute(element, false);
 
-    Location toLocation(SearchResult result) {
+    Location? toLocation(SearchResult result) {
       final lineInfo = server.getLineInfo(result.location.file);
       return searchResultToLocation(result, lineInfo);
     }
 
-    final referenceResults = convert(results, toLocation).toList();
+    final referenceResults =
+        convert(results, toLocation).whereNotNull().toList();
 
-    if (params.context?.includeDeclaration == true) {
+    final compilationUnit = unit.unit;
+    if (compilationUnit != null && params.context.includeDeclaration == true) {
       // Also include the definition for the symbol at this location.
-      referenceResults.addAll(_getDeclarations(unit.unit, offset));
+      referenceResults.addAll(_getDeclarations(compilationUnit, offset));
     }
 
     return success(referenceResults);
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_reject.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_reject.dart
index e841a61..dbcefeb 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_reject.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_reject.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/handlers/handlers.dart';
@@ -11,7 +9,7 @@
 
 /// A [MessageHandler] that rejects specific tpyes of messages with a given
 /// error code/message.
-class RejectMessageHandler extends MessageHandler<Object, void> {
+class RejectMessageHandler extends MessageHandler<Object?, void> {
   @override
   final Method handlesMessage;
   final ErrorCodes errorCode;
@@ -21,7 +19,7 @@
       : super(server);
 
   @override
-  LspJsonHandler<void> get jsonHandler => NullJsonHandler;
+  LspJsonHandler<Object?> get jsonHandler => NullJsonHandler;
 
   @override
   ErrorOr<void> handle(void _, CancellationToken token) {
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_rename.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_rename.dart
index caa7c78..6773974 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_rename.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_rename.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/constants.dart';
@@ -13,7 +11,7 @@
 import 'package:analysis_server/src/services/refactoring/refactoring.dart';
 
 class PrepareRenameHandler
-    extends MessageHandler<TextDocumentPositionParams, RangeAndPlaceholder> {
+    extends MessageHandler<TextDocumentPositionParams, RangeAndPlaceholder?> {
   PrepareRenameHandler(LspAnalysisServer server) : super(server);
   @override
   Method get handlesMessage => Method.textDocument_prepareRename;
@@ -23,7 +21,7 @@
       TextDocumentPositionParams.jsonHandler;
 
   @override
-  Future<ErrorOr<RangeAndPlaceholder>> handle(
+  Future<ErrorOr<RangeAndPlaceholder?>> handle(
       TextDocumentPositionParams params, CancellationToken token) async {
     if (!isDartDocument(params.textDocument)) {
       return success(null);
@@ -43,6 +41,10 @@
 
       final refactorDetails =
           RenameRefactoring.getElementToRename(node, element);
+      if (refactorDetails == null) {
+        return success(null);
+      }
+
       final refactoring = RenameRefactoring.create(
           server.refactoringWorkspace, unit.result, refactorDetails.element);
       if (refactoring == null) {
@@ -53,7 +55,7 @@
       final initStatus = await refactoring.checkInitialConditions();
       if (initStatus.hasFatalError) {
         return error(
-            ServerErrorCodes.RenameNotValid, initStatus.problem.message, null);
+            ServerErrorCodes.RenameNotValid, initStatus.problem!.message, null);
       }
 
       return success(RangeAndPlaceholder(
@@ -72,7 +74,7 @@
   }
 }
 
-class RenameHandler extends MessageHandler<RenameParams, WorkspaceEdit> {
+class RenameHandler extends MessageHandler<RenameParams, WorkspaceEdit?> {
   RenameHandler(LspAnalysisServer server) : super(server);
 
   @override
@@ -82,7 +84,7 @@
   LspJsonHandler<RenameParams> get jsonHandler => RenameParams.jsonHandler;
 
   @override
-  Future<ErrorOr<WorkspaceEdit>> handle(
+  Future<ErrorOr<WorkspaceEdit?>> handle(
       RenameParams params, CancellationToken token) async {
     if (!isDartDocument(params.textDocument)) {
       return success(null);
@@ -116,6 +118,10 @@
 
       final refactorDetails =
           RenameRefactoring.getElementToRename(node, element);
+      if (refactorDetails == null) {
+        return success(null);
+      }
+
       final refactoring = RenameRefactoring.create(
           server.refactoringWorkspace, unit.result, refactorDetails.element);
       if (refactoring == null) {
@@ -135,7 +141,7 @@
       }
       if (initStatus.hasFatalError) {
         return error(
-            ServerErrorCodes.RenameNotValid, initStatus.problem.message, null);
+            ServerErrorCodes.RenameNotValid, initStatus.problem!.message, null);
       }
 
       // Check the name is valid.
@@ -143,7 +149,7 @@
       final optionsStatus = refactoring.checkNewName();
       if (optionsStatus.hasError) {
         return error(ServerErrorCodes.RenameNotValid,
-            optionsStatus.problem.message, null);
+            optionsStatus.problem!.message, null);
       }
 
       // Final validation.
@@ -152,13 +158,13 @@
         return cancelled();
       }
       if (finalStatus.hasFatalError) {
-        return error(
-            ServerErrorCodes.RenameNotValid, finalStatus.problem.message, null);
+        return error(ServerErrorCodes.RenameNotValid,
+            finalStatus.problem!.message, null);
       } else if (finalStatus.hasError || finalStatus.hasWarning) {
         // Ask the user whether to proceed with the rename.
         final userChoice = await server.showUserPrompt(
           MessageType.Warning,
-          finalStatus.message,
+          finalStatus.message!,
           [
             MessageActionItem(title: UserPromptActions.renameAnyway),
             MessageActionItem(title: UserPromptActions.cancel),
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_semantic_tokens.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_semantic_tokens.dart
index 61e411e..7552f3f 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_semantic_tokens.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_semantic_tokens.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'dart:async';
 
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
@@ -18,7 +16,7 @@
 import 'package:analyzer_plugin/protocol/protocol_common.dart';
 
 abstract class AbstractSemanticTokensHandler<T>
-    extends MessageHandler<T, SemanticTokens>
+    extends MessageHandler<T, SemanticTokens?>
     with LspPluginRequestHandlerMixin {
   AbstractSemanticTokensHandler(LspAnalysisServer server) : super(server);
 
@@ -28,17 +26,18 @@
   }
 
   Future<List<SemanticTokenInfo>> getServerResult(
-      String path, SourceRange range) async {
+      String path, SourceRange? range) async {
     final result = await server.getResolvedUnit(path);
-    if (result?.state == ResultState.VALID) {
-      final computer = DartUnitHighlightsComputer(result.unit, range: range);
+    final unit = result?.unit;
+    if (result?.state == ResultState.VALID && unit != null) {
+      final computer = DartUnitHighlightsComputer(unit, range: range);
       return computer.computeSemanticTokens();
     }
     return [];
   }
 
   Iterable<SemanticTokenInfo> _filter(
-      Iterable<SemanticTokenInfo> tokens, SourceRange range) {
+      Iterable<SemanticTokenInfo> tokens, SourceRange? range) {
     if (range == null) {
       return tokens;
     }
@@ -48,9 +47,9 @@
             token.offset > range.end));
   }
 
-  Future<ErrorOr<SemanticTokens>> _handleImpl(
+  Future<ErrorOr<SemanticTokens?>> _handleImpl(
       TextDocumentIdentifier textDocument, CancellationToken token,
-      {Range range}) async {
+      {Range? range}) async {
     final path = pathOfDoc(textDocument);
 
     return path.mapResult((path) async {
@@ -61,7 +60,7 @@
         return success(null);
       }
 
-      return toSourceRange(lineInfo, range).mapResult((range) async {
+      return toSourceRangeNullable(lineInfo, range).mapResult((range) async {
         final serverTokens = await getServerResult(path, range);
         final pluginHighlightRegions =
             getPluginResults(path).expand((results) => results).toList();
@@ -125,7 +124,7 @@
       SemanticTokensParams.jsonHandler;
 
   @override
-  Future<ErrorOr<SemanticTokens>> handle(
+  Future<ErrorOr<SemanticTokens?>> handle(
           SemanticTokensParams params, CancellationToken token) =>
       _handleImpl(params.textDocument, token);
 }
@@ -142,7 +141,7 @@
       SemanticTokensRangeParams.jsonHandler;
 
   @override
-  Future<ErrorOr<SemanticTokens>> handle(
+  Future<ErrorOr<SemanticTokens?>> handle(
           SemanticTokensRangeParams params, CancellationToken token) =>
       _handleImpl(params.textDocument, token, range: params.range);
 }
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_shutdown.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_shutdown.dart
index a94c410..b226c29 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_shutdown.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_shutdown.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/handlers/handler_states.dart';
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_signature_help.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_signature_help.dart
index 3dd708c..b57381e 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_signature_help.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_signature_help.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/computer/computer_signature.dart';
@@ -12,7 +10,7 @@
 import 'package:analysis_server/src/lsp/mapping.dart';
 
 class SignatureHelpHandler
-    extends MessageHandler<SignatureHelpParams, SignatureHelp> {
+    extends MessageHandler<SignatureHelpParams, SignatureHelp?> {
   SignatureHelpHandler(LspAnalysisServer server) : super(server);
   @override
   Method get handlesMessage => Method.textDocument_signatureHelp;
@@ -22,12 +20,19 @@
       SignatureHelpParams.jsonHandler;
 
   @override
-  Future<ErrorOr<SignatureHelp>> handle(
+  Future<ErrorOr<SignatureHelp?>> handle(
       SignatureHelpParams params, CancellationToken token) async {
     if (!isDartDocument(params.textDocument)) {
       return success(null);
     }
 
+    final clientCapabilities = server.clientCapabilities;
+    if (clientCapabilities == null) {
+      // This should not happen unless a client misbehaves.
+      return error(ErrorCodes.ServerNotInitialized,
+          'Requests not before server is initilized');
+    }
+
     // If triggered automatically by pressing the trigger character, we will
     // only provide results if the character we typed was the one that actually
     // starts the argument list. This is to avoid popping open signature help
@@ -49,7 +54,7 @@
     return offset.mapResult((offset) {
       final computer = DartUnitSignatureComputer(
           server.getDartdocDirectiveInfoFor(unit.result),
-          unit.result.unit,
+          unit.result.unit!,
           offset);
       if (!computer.offsetIsValid) {
         return success(null); // No error, just no valid hover.
@@ -63,14 +68,11 @@
       // argument list.
       // The ArgumentList's offset is before the paren, but the request offset
       // will be after.
-      if (autoTriggered &&
-          computer.argumentList != null &&
-          offset != computer.argumentList.offset + 1) {
+      if (autoTriggered && offset != computer.argumentList.offset + 1) {
         return success(null);
       }
 
-      final formats =
-          server.clientCapabilities.signatureHelpDocumentationFormats;
+      final formats = clientCapabilities.signatureHelpDocumentationFormats;
       return success(toSignatureHelp(formats, signature));
     });
   }
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_states.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_states.dart
index 27041af..c8b201d 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_states.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_states.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'dart:async';
 
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
@@ -48,7 +46,7 @@
   FailureStateMessageHandler(LspAnalysisServer server) : super(server);
 
   @override
-  FutureOr<ErrorOr<Object>> handleUnknownMessage(IncomingMessage message) {
+  FutureOr<ErrorOr<Object?>> handleUnknownMessage(IncomingMessage message) {
     return error(
         ErrorCodes.InternalError,
         'An unrecoverable error occurred and the server cannot process messages',
@@ -126,7 +124,7 @@
   }
 
   @override
-  ErrorOr<void> handleUnknownMessage(IncomingMessage message) {
+  ErrorOr<Object?> handleUnknownMessage(IncomingMessage message) {
     // Silently drop non-requests.
     if (message is! RequestMessage) {
       server.instrumentationService
@@ -146,7 +144,7 @@
   }
 
   @override
-  FutureOr<ErrorOr<Object>> handleUnknownMessage(IncomingMessage message) {
+  FutureOr<ErrorOr<Object?>> handleUnknownMessage(IncomingMessage message) {
     // Silently drop non-requests.
     if (message is! RequestMessage) {
       server.instrumentationService
@@ -166,7 +164,7 @@
   }
 
   @override
-  FutureOr<ErrorOr<Object>> handleUnknownMessage(IncomingMessage message) {
+  FutureOr<ErrorOr<Object?>> handleUnknownMessage(IncomingMessage message) {
     // Silently drop non-requests.
     if (message is! RequestMessage) {
       server.instrumentationService
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_text_document_changes.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_text_document_changes.dart
index a03eb33..003917f 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_text_document_changes.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_text_document_changes.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
+import 'dart:async';
 
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
@@ -23,14 +23,15 @@
       DidChangeTextDocumentParams.jsonHandler;
 
   @override
-  ErrorOr<Null> handle(
+  FutureOr<ErrorOr<Null>> handle(
       DidChangeTextDocumentParams params, CancellationToken token) {
     final path = pathOfDoc(params.textDocument);
     return path.mapResult((path) => _changeFile(path, params));
   }
 
-  ErrorOr<Null> _changeFile(String path, DidChangeTextDocumentParams params) {
-    String oldContents;
+  FutureOr<ErrorOr<Null>> _changeFile(
+      String path, DidChangeTextDocumentParams params) {
+    String? oldContents;
     if (server.resourceProvider.hasOverlay(path)) {
       oldContents = server.resourceProvider.getFile(path).readAsStringSync();
     }
@@ -66,7 +67,7 @@
       DidCloseTextDocumentParams.jsonHandler;
 
   @override
-  ErrorOr<Null> handle(
+  FutureOr<ErrorOr<Null>> handle(
       DidCloseTextDocumentParams params, CancellationToken token) {
     final path = pathOfDoc(params.textDocument);
     return path.mapResult((path) {
@@ -91,7 +92,7 @@
       DidOpenTextDocumentParams.jsonHandler;
 
   @override
-  ErrorOr<Null> handle(
+  FutureOr<ErrorOr<Null>> handle(
       DidOpenTextDocumentParams params, CancellationToken token) {
     final doc = params.textDocument;
     final path = pathOfDocItem(doc);
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_will_rename_files.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_will_rename_files.dart
index c256d6d..d08152f 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_will_rename_files.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_will_rename_files.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/handlers/handlers.dart';
@@ -11,7 +9,8 @@
 import 'package:analysis_server/src/lsp/mapping.dart';
 import 'package:analysis_server/src/services/refactoring/refactoring.dart';
 
-class WillRenameFilesHandler extends MessageHandler<RenameFilesParams, void> {
+class WillRenameFilesHandler
+    extends MessageHandler<RenameFilesParams, WorkspaceEdit?> {
   WillRenameFilesHandler(LspAnalysisServer server) : super(server);
   @override
   Method get handlesMessage => Method.workspace_willRenameFiles;
@@ -21,9 +20,9 @@
       RenameFilesParams.jsonHandler;
 
   @override
-  Future<ErrorOr<WorkspaceEdit>> handle(
+  Future<ErrorOr<WorkspaceEdit?>> handle(
       RenameFilesParams params, CancellationToken token) async {
-    final files = params?.files ?? [];
+    final files = params.files;
     // For performance reasons, only single-file rename/moves are currently supported.
     if (files.length > 1 || files.any((f) => !f.oldUri.endsWith('.dart'))) {
       return success(null);
@@ -36,7 +35,7 @@
         newPath.mapResult((newPath) => _renameFile(oldPath, newPath)));
   }
 
-  Future<ErrorOr<WorkspaceEdit>> _renameFile(
+  Future<ErrorOr<WorkspaceEdit?>> _renameFile(
       String oldPath, String newPath) async {
     final resolvedUnit = await server.getResolvedUnit(oldPath);
     if (resolvedUnit == null) {
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_workspace_configuration.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_workspace_configuration.dart
index 067641c..36d5372 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_workspace_configuration.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_workspace_configuration.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/handlers/handlers.dart';
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handler_workspace_symbols.dart b/pkg/analysis_server/lib/src/lsp/handlers/handler_workspace_symbols.dart
index 37f455b..96a2cae 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handler_workspace_symbols.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handler_workspace_symbols.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/handlers/handlers.dart';
@@ -24,15 +22,25 @@
   @override
   Future<ErrorOr<List<SymbolInformation>>> handle(
       WorkspaceSymbolParams params, CancellationToken token) async {
+    final clientCapabilities = server.clientCapabilities;
+    if (clientCapabilities == null) {
+      // This should not happen unless a client misbehaves.
+      return error(ErrorCodes.ServerNotInitialized,
+          'Requests not before server is initilized');
+    }
+
     // Respond to empty queries with an empty list. The spec says this should
     // be non-empty, however VS Code's client sends empty requests (but then
     // appears to not render the results we supply anyway).
-    final query = params?.query ?? '';
+    // TODO(dantup): The spec has been updated to allow empty queries. Clients
+    // may expect a full list in this case, though we may choose not to send
+    // it on performance grounds until they type a filter.
+    final query = params.query;
     if (query == '') {
       return success([]);
     }
 
-    final supportedSymbolKinds = server.clientCapabilities.workspaceSymbolKinds;
+    final supportedSymbolKinds = clientCapabilities.workspaceSymbolKinds;
 
     // Convert the string input into a case-insensitive regex that has wildcards
     // between every character and at start/end to allow for fuzzy matching.
@@ -45,7 +53,7 @@
     var remainingResults = 500;
 
     final filePathsHashSet = <String>{};
-    final tracker = server.declarationsTracker;
+    final tracker = server.declarationsTracker!;
     final declarations = search.WorkspaceSymbols(tracker).declarations(
       regex,
       remainingResults,
@@ -89,10 +97,9 @@
       range: range,
     );
 
-    final hasParameters =
-        declaration.parameters != null && declaration.parameters.isNotEmpty;
-    final nameSuffix =
-        hasParameters ? (declaration.parameters == '()' ? '()' : '(…)') : '';
+    final parameters = declaration.parameters;
+    final hasParameters = parameters != null && parameters.isNotEmpty;
+    final nameSuffix = hasParameters ? (parameters == '()' ? '()' : '(…)') : '';
 
     return SymbolInformation(
         name: '${declaration.name}$nameSuffix',
diff --git a/pkg/analysis_server/lib/src/lsp/handlers/handlers.dart b/pkg/analysis_server/lib/src/lsp/handlers/handlers.dart
index 4a26e3c..6e31bf2 100644
--- a/pkg/analysis_server/lib/src/lsp/handlers/handlers.dart
+++ b/pkg/analysis_server/lib/src/lsp/handlers/handlers.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'dart:async';
 
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
@@ -46,21 +44,22 @@
 }
 
 abstract class CommandHandler<P, R> with Handler<P, R> {
-  CommandHandler(LspAnalysisServer server) {
-    this.server = server;
-  }
+  @override
+  final LspAnalysisServer server;
 
-  Future<ErrorOr<void>> handle(List<dynamic> arguments,
+  CommandHandler(this.server);
+
+  Future<ErrorOr<Object?>> handle(List<dynamic>? arguments,
       ProgressReporter progress, CancellationToken cancellationToken);
 }
 
 mixin Handler<P, R> {
-  LspAnalysisServer server;
-
   final fileModifiedError = error<R>(ErrorCodes.ContentModified,
       'Document was modified before operation completed', null);
 
-  bool fileHasBeenModified(String path, int clientVersion) {
+  LspAnalysisServer get server;
+
+  bool fileHasBeenModified(String path, num? clientVersion) {
     final serverDocIdentifier = server.getVersionedDocumentIdentifier(path);
     return clientVersion != null &&
         clientVersion != serverDocIdentifier.version;
@@ -81,7 +80,7 @@
     if (result?.state != ResultState.VALID) {
       return error(ServerErrorCodes.InvalidFilePath, 'Invalid file path', path);
     }
-    return success(result);
+    return success(result!);
   }
 
   ErrorOr<ParsedUnitResult> requireUnresolvedUnit(String path) {
@@ -89,7 +88,7 @@
     if (result?.state != ResultState.VALID) {
       return error(ServerErrorCodes.InvalidFilePath, 'Invalid file path', path);
     }
-    return success(result);
+    return success(result!);
   }
 }
 
@@ -103,7 +102,7 @@
     final driver = server.getAnalysisDriver(path);
     final pluginFutures = server.pluginManager.broadcastRequest(
       params,
-      contextRoot: driver.analysisContext.contextRoot,
+      contextRoot: driver?.analysisContext?.contextRoot,
     );
 
     return waitForResponses(pluginFutures,
@@ -116,9 +115,10 @@
 /// Clients may not extend, implement or mix-in this class.
 abstract class MessageHandler<P, R>
     with Handler<P, R>, RequestHandlerMixin<LspAnalysisServer> {
-  MessageHandler(LspAnalysisServer server) {
-    this.server = server;
-  }
+  @override
+  final LspAnalysisServer server;
+
+  MessageHandler(this.server);
 
   /// The method that this handler can handle.
   Method get handlesMessage;
@@ -169,7 +169,7 @@
   /// Handle the given [message]. If the [message] is a [RequestMessage], then the
   /// return value will be sent back in a [ResponseMessage].
   /// [NotificationMessage]s are not expected to return results.
-  FutureOr<ErrorOr<Object>> handleMessage(IncomingMessage message) async {
+  FutureOr<ErrorOr<Object?>> handleMessage(IncomingMessage message) async {
     final handler = _messageHandlers[message.method];
     if (handler == null) {
       return handleUnknownMessage(message);
@@ -197,7 +197,7 @@
     }
   }
 
-  FutureOr<ErrorOr<Object>> handleUnknownMessage(IncomingMessage message) {
+  FutureOr<ErrorOr<Object?>> handleUnknownMessage(IncomingMessage message) {
     // If it's an optional *Notification* we can ignore it (return success).
     // Otherwise respond with failure. Optional Requests must still be responded
     // to so they don't leave open requests on the client.
@@ -207,11 +207,6 @@
   }
 
   void registerHandler(MessageHandler handler) {
-    assert(
-        handler.handlesMessage != null,
-        'Unable to register handler ${handler.runtimeType} because it does '
-        'not declare which messages it can handle');
-
     _messageHandlers[handler.handlesMessage] = handler;
   }
 
diff --git a/pkg/analysis_server/lib/src/lsp/lsp_analysis_server.dart b/pkg/analysis_server/lib/src/lsp/lsp_analysis_server.dart
index c987a3f..1447e55 100644
--- a/pkg/analysis_server/lib/src/lsp/lsp_analysis_server.dart
+++ b/pkg/analysis_server/lib/src/lsp/lsp_analysis_server.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'dart:async';
 
 import 'package:analysis_server/lsp_protocol/protocol_custom_generated.dart';
@@ -59,11 +57,11 @@
 /// them.
 class LspAnalysisServer extends AbstractAnalysisServer {
   /// The capabilities of the LSP client. Will be null prior to initialization.
-  LspClientCapabilities _clientCapabilities;
+  LspClientCapabilities? _clientCapabilities;
 
   /// Initialization options provided by the LSP client. Allows opting in/out of
   /// specific server functionality. Will be null prior to initialization.
-  LspInitializationOptions _initializationOptions;
+  LspInitializationOptions? _initializationOptions;
 
   /// Configuration for the workspace from the client. This is similar to
   /// initializationOptions but can be updated dynamically rather than set
@@ -76,7 +74,7 @@
 
   /// The workspace for rename refactorings. Should be accessed through the
   /// refactoringWorkspace getter to be automatically created (lazily).
-  RefactoringWorkspace _refactoringWorkspace;
+  RefactoringWorkspace? _refactoringWorkspace;
 
   /// The versions of each document known to the server (keyed by path), used to
   /// send back to the client for server-initiated edits so that the client can
@@ -87,7 +85,7 @@
   /// not known.
   final Map<String, VersionedTextDocumentIdentifier> documentVersions = {};
 
-  ServerStateMessageHandler messageHandler;
+  late ServerStateMessageHandler messageHandler;
 
   int nextRequestId = 1;
 
@@ -95,8 +93,8 @@
 
   /// Capabilities of the server. Will be null prior to initialization as
   /// the server capabilities depend on the client capabilities.
-  ServerCapabilities capabilities;
-  ServerCapabilitiesComputer capabilitiesComputer;
+  ServerCapabilities? capabilities;
+  late ServerCapabilitiesComputer capabilitiesComputer;
 
   LspPerformance performanceStats = LspPerformance();
 
@@ -104,13 +102,13 @@
   /// automatically.
   bool willExit = false;
 
-  StreamSubscription _pluginChangeSubscription;
+  StreamSubscription? _pluginChangeSubscription;
 
   /// The current workspace folders provided by the client. Used as analysis roots.
   final _workspaceFolders = <String>{};
 
   /// A progress reporter for analysis status.
-  ProgressReporter analyzingProgressReporter;
+  ProgressReporter? analyzingProgressReporter;
 
   /// The number of times contexts have been created/recreated.
   @visibleForTesting
@@ -125,8 +123,8 @@
     DartSdkManager sdkManager,
     CrashReportingAttachmentsBuilder crashReportingAttachmentsBuilder,
     InstrumentationService instrumentationService, {
-    http.Client httpClient,
-    DiagnosticServer diagnosticServer,
+    http.Client? httpClient,
+    DiagnosticServer? diagnosticServer,
     // Disable to avoid using this in unit tests.
     bool enableBazelWatcher = false,
   }) : super(
@@ -157,16 +155,18 @@
   }
 
   /// The capabilities of the LSP client. Will be null prior to initialization.
-  LspClientCapabilities get clientCapabilities => _clientCapabilities;
+  LspClientCapabilities? get clientCapabilities => _clientCapabilities;
 
   Future<void> get exited => channel.closed;
 
   /// Initialization options provided by the LSP client. Allows opting in/out of
   /// specific server functionality. Will be null prior to initialization.
-  LspInitializationOptions get initializationOptions => _initializationOptions;
+  LspInitializationOptions get initializationOptions =>
+      _initializationOptions as LspInitializationOptions;
 
   @override
-  LspNotificationManager get notificationManager => super.notificationManager;
+  LspNotificationManager get notificationManager =>
+      super.notificationManager as LspNotificationManager;
 
   @override
   set pluginManager(PluginManager value) {
@@ -202,7 +202,7 @@
   /// Fetches configuration from the client (if supported) and then sends
   /// register/unregister requests for any supported/enabled dynamic registrations.
   Future<void> fetchClientConfigurationAndPerformDynamicRegistration() async {
-    if (clientCapabilities.configuration) {
+    if (clientCapabilities?.configuration ?? false) {
       // Fetch all configuration we care about from the client. This is just
       // "dart" for now, but in future this may be extended to include
       // others (for example "flutter").
@@ -243,8 +243,8 @@
   /// Return the LineInfo for the file with the given [path]. The file is
   /// analyzed in one of the analysis drivers to which the file was added,
   /// otherwise in the first driver, otherwise `null` is returned.
-  LineInfo getLineInfo(String path) {
-    return getAnalysisDriver(path)?.getFileSync(path)?.lineInfo;
+  LineInfo? getLineInfo(String path) {
+    return getAnalysisDriver(path)?.getFileSync(path).lineInfo;
   }
 
   /// Gets the version of a document known to the server, returning a
@@ -263,16 +263,22 @@
     _initializationOptions = LspInitializationOptions(initializationOptions);
 
     performanceAfterStartup = ServerPerformance();
-    performance = performanceAfterStartup;
+    performance = performanceAfterStartup!;
   }
 
   /// Handles a response from the client by invoking the completer that the
   /// outbound request created.
   void handleClientResponse(ResponseMessage message) {
-    // The ID from the client is an Either2<num, String>, though it's not valid
-    // for it to be a string because it should match a request we sent to the
-    // client (and we always use numeric IDs for outgoing requests).
-    message.id.map(
+    // The ID from the client is an Either2<num, String>?, though it's not valid
+    // for it to be a null or a string because it should match a request we sent
+    // to the client (and we always use numeric IDs for outgoing requests).
+    final id = message.id;
+    if (id == null) {
+      showErrorMessageToUser('Unexpected response with no ID!');
+      return;
+    }
+
+    id.map(
       (id) {
         // It's possible that even if we got a numeric ID that it's not valid.
         // If it's not in our completers list (which is a list of the
@@ -366,7 +372,7 @@
     exceptions.add(ServerException(
       message,
       exception,
-      stackTrace is StackTrace ? stackTrace : null,
+      stackTrace is StackTrace ? stackTrace : StackTrace.current,
       false,
     ));
 
@@ -399,8 +405,8 @@
   ///
   /// If the result of applying the edits is already known, [newContent] can be
   /// set to avoid doing that calculation twice.
-  void onOverlayUpdated(String path, Iterable<plugin.SourceEdit> edits,
-      {String newContent}) {
+  void onOverlayUpdated(String path, List<plugin.SourceEdit> edits,
+      {String? newContent}) {
     assert(resourceProvider.hasOverlay(path));
     if (newContent == null) {
       final oldContent = resourceProvider.getFile(path).readAsStringSync();
@@ -538,7 +544,7 @@
     // Send old custom notifications to clients that do not support $/progress.
     // TODO(dantup): Remove this custom notification (and related classes) when
     // it's unlikely to be in use by any clients.
-    if (clientCapabilities.workDoneProgress != true) {
+    if (clientCapabilities?.workDoneProgress != true) {
       channel.sendNotification(NotificationMessage(
         method: CustomMethods.analyzerStatus,
         params: AnalyzerStatusParams(isAnalyzing: status.isAnalyzing),
@@ -555,7 +561,7 @@
       if (analyzingProgressReporter != null) {
         // Do not null this out until after end completes, otherwise we may try
         // to create a new token before it's really completed.
-        await analyzingProgressReporter.end();
+        await analyzingProgressReporter?.end();
         analyzingProgressReporter = null;
       }
     }
@@ -634,8 +640,8 @@
     // TODO(dantup): This is currently case-sensitive!
 
     _workspaceFolders
-      ..addAll(addedPaths ?? const [])
-      ..removeAll(removedPaths ?? const []);
+      ..addAll(addedPaths)
+      ..removeAll(removedPaths);
 
     _refreshAnalysisRoots();
   }
@@ -802,25 +808,35 @@
 
   @override
   void listenAnalysisDriver(nd.AnalysisDriver analysisDriver) {
-    analysisServer.declarationsTracker
-        ?.addContext(analysisDriver.analysisContext);
+    // TODO(dantup): Is this required, or covered by
+    // addContextsToDeclarationsTracker? The original server does not appear to
+    // have an equivalent call.
+    final analysisContext = analysisDriver.analysisContext;
+    if (analysisContext != null) {
+      analysisServer.declarationsTracker?.addContext(analysisContext);
+    }
 
     analysisDriver.results.listen((result) {
       var path = result.path;
+      if (path == null) {
+        // This shouldn't occur - result.path is marked with a TODO to become
+        // non-nullable.
+        return;
+      }
       filesToFlush.add(path);
       if (analysisServer.isAnalyzed(path)) {
         final serverErrors = protocol.doAnalysisError_listFromEngine(result);
-        recordAnalysisErrors(result.path, serverErrors);
+        recordAnalysisErrors(path, serverErrors);
       }
-      if (result.unit != null) {
+      final unit = result.unit;
+      if (unit != null) {
         if (analysisServer.shouldSendClosingLabelsFor(path)) {
-          final labels =
-              DartUnitClosingLabelsComputer(result.lineInfo, result.unit)
-                  .compute()
-                  .map((l) => toClosingLabel(result.lineInfo, l))
-                  .toList();
+          final labels = DartUnitClosingLabelsComputer(result.lineInfo, unit)
+              .compute()
+              .map((l) => toClosingLabel(result.lineInfo, l))
+              .toList();
 
-          analysisServer.publishClosingLabels(result.path, labels);
+          analysisServer.publishClosingLabels(path, labels);
         }
         if (analysisServer.shouldSendOutlineFor(path)) {
           final outline = DartUnitOutlineComputer(
@@ -828,12 +844,12 @@
             withBasicFlutter: true,
           ).compute();
           final lspOutline = toOutline(result.lineInfo, outline);
-          analysisServer.publishOutline(result.path, lspOutline);
+          analysisServer.publishOutline(path, lspOutline);
         }
         if (analysisServer.shouldSendFlutterOutlineFor(path)) {
           final outline = FlutterOutlineComputer(result).compute();
           final lspOutline = toFlutterOutline(result.lineInfo, outline);
-          analysisServer.publishFlutterOutline(result.path, lspOutline);
+          analysisServer.publishFlutterOutline(path, lspOutline);
         }
       }
     });
diff --git a/pkg/analysis_server/lib/src/lsp/lsp_socket_server.dart b/pkg/analysis_server/lib/src/lsp/lsp_socket_server.dart
index d567d37..25c012e 100644
--- a/pkg/analysis_server/lib/src/lsp/lsp_socket_server.dart
+++ b/pkg/analysis_server/lib/src/lsp/lsp_socket_server.dart
@@ -1,7 +1,7 @@
 // Copyright (c) 2018, the Dart project authors. Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
-
+//
 // @dart = 2.9
 
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
diff --git a/pkg/analysis_server/lib/src/lsp/mapping.dart b/pkg/analysis_server/lib/src/lsp/mapping.dart
index 3f0b00c..f9d8355 100644
--- a/pkg/analysis_server/lib/src/lsp/mapping.dart
+++ b/pkg/analysis_server/lib/src/lsp/mapping.dart
@@ -2,12 +2,11 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'dart:math';
 
 import 'package:analysis_server/lsp_protocol/protocol_custom_generated.dart'
     as lsp;
+import 'package:analysis_server/lsp_protocol/protocol_custom_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart' as lsp;
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
@@ -32,7 +31,7 @@
 import 'package:analyzer/src/services/available_declarations.dart' as dec;
 import 'package:analyzer_plugin/protocol/protocol_common.dart' as plugin;
 import 'package:analyzer_plugin/utilities/pair.dart';
-import 'package:meta/meta.dart';
+import 'package:collection/collection.dart';
 
 const languageSourceName = 'dart';
 
@@ -51,11 +50,7 @@
 };
 
 lsp.Either2<String, lsp.MarkupContent> asStringOrMarkupContent(
-    Set<lsp.MarkupKind> preferredFormats, String content) {
-  if (content == null) {
-    return null;
-  }
-
+    Set<lsp.MarkupKind>? preferredFormats, String content) {
   return preferredFormats == null
       ? lsp.Either2<String, lsp.MarkupContent>.t1(content)
       : lsp.Either2<String, lsp.MarkupContent>.t2(
@@ -64,8 +59,8 @@
 
 /// Builds an LSP snippet string with supplied ranges as tabstops.
 String buildSnippetStringWithTabStops(
-  String text,
-  List<int> offsetLengthPairs,
+  String? text,
+  List<int>? offsetLengthPairs,
 ) {
   text ??= '';
   offsetLengthPairs ??= const [];
@@ -119,11 +114,13 @@
 lsp.WorkspaceEdit createPlainWorkspaceEdit(
     lsp.LspAnalysisServer server, List<server.SourceFileEdit> edits) {
   return toWorkspaceEdit(
-      server.clientCapabilities,
+      // Client capabilities are always available after initialization.
+      server.clientCapabilities!,
       edits
           .map((e) => FileEditInformation(
                 server.getVersionedDocumentIdentifier(e.file),
-                server.getLineInfo(e.file),
+                // We should never produce edits for a file with no LineInfo.
+                server.getLineInfo(e.file)!,
                 e.edits,
                 // fileStamp == 1 is used by the server to indicate the file needs creating.
                 newFile: e.fileStamp == -1,
@@ -143,7 +140,7 @@
   // In order to return snippets, we must ensure we are only modifying a single
   // existing file with a single edit and that there is a linked edit group with
   // only one position and no suggestions.
-  if (!server.clientCapabilities.experimentalSnippetTextEdit ||
+  if (!server.clientCapabilities!.experimentalSnippetTextEdit ||
       change.edits.length != 1 ||
       change.edits.first.fileStamp == -1 || // new file
       change.edits.first.edits.length != 1 ||
@@ -164,11 +161,12 @@
   }
 
   return toWorkspaceEdit(
-      server.clientCapabilities,
+      server.clientCapabilities!,
       change.edits
           .map((e) => FileEditInformation(
                 server.getVersionedDocumentIdentifier(e.file),
-                server.getLineInfo(e.file),
+                // We should never produce edits for a file with no LineInfo.
+                server.getLineInfo(e.file)!,
                 e.edits,
                 selectionOffsetRelative: selectionOffset - edit.offset,
                 selectionLength: selectionLength,
@@ -177,7 +175,7 @@
           .toList());
 }
 
-lsp.CompletionItemKind declarationKindToCompletionItemKind(
+lsp.CompletionItemKind? declarationKindToCompletionItemKind(
   Set<lsp.CompletionItemKind> supportedCompletionKinds,
   dec.DeclarationKind kind,
 ) {
@@ -210,12 +208,12 @@
     }
   }
 
-  return getKindPreferences().firstWhere(isSupported, orElse: () => null);
+  return getKindPreferences().firstWhereOrNull(isSupported);
 }
 
 lsp.SymbolKind declarationKindToSymbolKind(
   Set<lsp.SymbolKind> supportedSymbolKinds,
-  server.DeclarationKind kind,
+  server.DeclarationKind? kind,
 ) {
   bool isSupported(lsp.SymbolKind kind) => supportedSymbolKinds.contains(kind);
 
@@ -274,26 +272,25 @@
   int replacementOffset,
   int insertLength,
   int replacementLength, {
-  @required bool includeCommitCharacters,
-  @required bool completeFunctionCalls,
+  required bool includeCommitCharacters,
+  required bool completeFunctionCalls,
 }) {
   final supportsSnippets = capabilities.completionSnippets;
+  final parent = declaration.parent;
 
   String completion;
   switch (declaration.kind) {
     case DeclarationKind.ENUM_CONSTANT:
-      completion = '${declaration.parent.name}.${declaration.name}';
+      completion = '${parent!.name}.${declaration.name}';
       break;
     case DeclarationKind.GETTER:
     case DeclarationKind.FIELD:
-      completion = declaration.parent != null &&
-              declaration.parent.name != null &&
-              declaration.parent.name.isNotEmpty
-          ? '${declaration.parent.name}.${declaration.name}'
+      completion = parent != null && parent.name.isNotEmpty
+          ? '${parent.name}.${declaration.name}'
           : declaration.name;
       break;
     case DeclarationKind.CONSTRUCTOR:
-      completion = declaration.parent.name;
+      completion = parent!.name;
       if (declaration.name.isNotEmpty) {
         completion += '.${declaration.name}';
       }
@@ -347,10 +344,8 @@
       capabilities.completionItemKinds, declaration.kind);
 
   var relevanceBoost = 0;
-  if (declaration.relevanceTags != null) {
-    declaration.relevanceTags.forEach(
-        (t) => relevanceBoost = max(relevanceBoost, tagBoosts[t] ?? 0));
-  }
+  declaration.relevanceTags
+      .forEach((t) => relevanceBoost = max(relevanceBoost, tagBoosts[t] ?? 0));
   final itemRelevance = includedSuggestionSet.relevance + relevanceBoost;
 
   // Because we potentially send thousands of these items, we should minimise
@@ -392,14 +387,14 @@
         file: file,
         offset: offset,
         libId: includedSuggestionSet.id,
-        displayUri: includedSuggestionSet.displayUri ?? library.uri?.toString(),
+        displayUri: includedSuggestionSet.displayUri ?? library.uri.toString(),
         rOffset: replacementOffset,
         iLength: insertLength,
         rLength: replacementLength),
   );
 }
 
-lsp.CompletionItemKind elementKindToCompletionItemKind(
+lsp.CompletionItemKind? elementKindToCompletionItemKind(
   Set<lsp.CompletionItemKind> supportedCompletionKinds,
   server.ElementKind kind,
 ) {
@@ -461,12 +456,12 @@
     }
   }
 
-  return getKindPreferences().firstWhere(isSupported, orElse: () => null);
+  return getKindPreferences().firstWhereOrNull(isSupported);
 }
 
 lsp.SymbolKind elementKindToSymbolKind(
   Set<lsp.SymbolKind> supportedSymbolKinds,
-  server.ElementKind kind,
+  server.ElementKind? kind,
 ) {
   bool isSupported(lsp.SymbolKind kind) => supportedSymbolKinds.contains(kind);
 
@@ -538,20 +533,21 @@
       .firstWhere(isSupported, orElse: () => lsp.SymbolKind.Obj);
 }
 
-String getCompletionDetail(
+String? getCompletionDetail(
   server.CompletionSuggestion suggestion,
-  lsp.CompletionItemKind completionKind,
+  lsp.CompletionItemKind? completionKind,
   bool supportsDeprecated,
 ) {
-  final hasElement = suggestion.element != null;
-  final hasParameters = hasElement &&
-      suggestion.element.parameters != null &&
-      suggestion.element.parameters.isNotEmpty;
-  final hasReturnType = hasElement &&
-      suggestion.element.returnType != null &&
-      suggestion.element.returnType.isNotEmpty;
-  final hasParameterType =
-      suggestion.parameterType != null && suggestion.parameterType.isNotEmpty;
+  final element = suggestion.element;
+  final hasElement = element != null;
+  final parameters = element?.parameters;
+  final returnType = element?.returnType;
+  final parameterType = suggestion.parameterType;
+  final hasParameters =
+      hasElement && parameters != null && parameters.isNotEmpty;
+  final hasReturnType =
+      hasElement && returnType != null && returnType.isNotEmpty;
+  final hasParameterType = parameterType != null && parameterType.isNotEmpty;
 
   final prefix =
       supportsDeprecated || !suggestion.isDeprecated ? '' : '(Deprecated) ';
@@ -562,36 +558,34 @@
     // To avoid this, always show only the return type, whether it's a getter
     // or a setter.
     return prefix +
-        (suggestion.element.kind == server.ElementKind.GETTER
-            ? suggestion.element.returnType
+        (element?.kind == server.ElementKind.GETTER
+            ? (returnType ?? '')
             // Don't assume setters always have parameters
             // See https://github.com/dart-lang/sdk/issues/27747
-            : suggestion.element.parameters != null &&
-                    suggestion.element.parameters.isNotEmpty
+            : parameters != null && parameters.isNotEmpty
                 // Extract the type part from '(MyType value)`
-                ? suggestion.element.parameters.substring(
-                    1, suggestion.element.parameters.lastIndexOf(' '))
+                ? parameters.substring(1, parameters.lastIndexOf(' '))
                 : '');
   } else if (hasParameters && hasReturnType) {
-    return '$prefix${suggestion.element.parameters} → ${suggestion.element.returnType}';
+    return '$prefix$parameters → $returnType';
   } else if (hasReturnType) {
-    return '$prefix${suggestion.element.returnType}';
+    return '$prefix$returnType';
   } else if (hasParameterType) {
-    return '$prefix${suggestion.parameterType}';
+    return '$prefix$parameterType';
   } else {
     return prefix.isNotEmpty ? prefix : null;
   }
 }
 
-String getDeclarationCompletionDetail(
+String? getDeclarationCompletionDetail(
   dec.Declaration declaration,
-  lsp.CompletionItemKind completionKind,
+  lsp.CompletionItemKind? completionKind,
   bool supportsDeprecated,
 ) {
-  final hasParameters =
-      declaration.parameters != null && declaration.parameters.isNotEmpty;
-  final hasReturnType =
-      declaration.returnType != null && declaration.returnType.isNotEmpty;
+  final parameters = declaration.parameters;
+  final hasParameters = parameters != null && parameters.isNotEmpty;
+  final returnType = declaration.returnType;
+  final hasReturnType = returnType != null && returnType.isNotEmpty;
 
   final prefix =
       supportsDeprecated || !declaration.isDeprecated ? '' : '(Deprecated) ';
@@ -603,15 +597,15 @@
     // or a setter.
     var suffix = '';
     if (declaration.kind == dec.DeclarationKind.GETTER) {
-      suffix = declaration.returnType;
+      suffix = declaration.returnType ?? '';
     } else {
       // Don't assume setters always have parameters
       // See https://github.com/dart-lang/sdk/issues/27747
-      if (declaration.parameters != null && declaration.parameters.isNotEmpty) {
+      if (parameters != null && parameters.isNotEmpty) {
         // Extract the type part from `(MyType value)`, if there is a type.
-        var spaceIndex = declaration.parameters.lastIndexOf(' ');
+        var spaceIndex = parameters.lastIndexOf(' ');
         if (spaceIndex > 0) {
-          suffix = declaration.parameters.substring(1, spaceIndex);
+          suffix = parameters.substring(1, spaceIndex);
         }
       }
     }
@@ -625,51 +619,45 @@
   }
 }
 
-List<lsp.DiagnosticTag> getDiagnosticTags(
-    Set<lsp.DiagnosticTag> supportedTags, plugin.AnalysisError error) {
+List<lsp.DiagnosticTag>? getDiagnosticTags(
+    Set<lsp.DiagnosticTag>? supportedTags, plugin.AnalysisError error) {
   if (supportedTags == null) {
     return null;
   }
 
   final tags = diagnosticTagsForErrorCode[error.code]
       ?.where(supportedTags.contains)
-      ?.toList();
+      .toList();
 
   return tags != null && tags.isNotEmpty ? tags : null;
 }
 
-bool isDartDocument(lsp.TextDocumentIdentifier doc) =>
-    doc?.uri?.endsWith('.dart');
+bool isDartDocument(lsp.TextDocumentIdentifier? doc) =>
+    doc?.uri.endsWith('.dart') ?? false;
 
 lsp.Location navigationTargetToLocation(
   String targetFilePath,
   server.NavigationTarget target,
   server.LineInfo targetLineInfo,
 ) {
-  if (targetLineInfo == null) {
-    return null;
-  }
-
   return lsp.Location(
     uri: Uri.file(targetFilePath).toString(),
     range: toRange(targetLineInfo, target.offset, target.length),
   );
 }
 
-lsp.LocationLink navigationTargetToLocationLink(
+lsp.LocationLink? navigationTargetToLocationLink(
   server.NavigationRegion region,
   server.LineInfo regionLineInfo,
   String targetFilePath,
   server.NavigationTarget target,
   server.LineInfo targetLineInfo,
 ) {
-  if (regionLineInfo == null || targetLineInfo == null) {
-    return null;
-  }
-
   final nameRange = toRange(targetLineInfo, target.offset, target.length);
-  final codeRange = target.codeOffset != null && target.codeLength != null
-      ? toRange(targetLineInfo, target.codeOffset, target.codeLength)
+  final codeOffset = target.codeOffset;
+  final codeLength = target.codeLength;
+  final codeRange = codeOffset != null && codeLength != null
+      ? toRange(targetLineInfo, codeOffset, codeLength)
       : nameRange;
 
   return lsp.LocationLink(
@@ -682,21 +670,21 @@
 
 /// Returns the file system path for a TextDocumentIdentifier.
 ErrorOr<String> pathOfDoc(lsp.TextDocumentIdentifier doc) =>
-    pathOfUri(Uri.tryParse(doc?.uri));
+    pathOfUri(Uri.tryParse(doc.uri));
 
 /// Returns the file system path for a TextDocumentItem.
 ErrorOr<String> pathOfDocItem(lsp.TextDocumentItem doc) =>
-    pathOfUri(Uri.tryParse(doc?.uri));
+    pathOfUri(Uri.tryParse(doc.uri));
 
 /// Returns the file system path for a file URI.
-ErrorOr<String> pathOfUri(Uri uri) {
+ErrorOr<String> pathOfUri(Uri? uri) {
   if (uri == null) {
     return ErrorOr<String>.error(ResponseError(
       code: lsp.ServerErrorCodes.InvalidFilePath,
       message: 'Document URI was not supplied',
     ));
   }
-  final isValidFileUri = uri?.isScheme('file') ?? false;
+  final isValidFileUri = uri.isScheme('file');
   if (!isValidFileUri) {
     return ErrorOr<String>.error(ResponseError(
       code: lsp.ServerErrorCodes.InvalidFilePath,
@@ -719,13 +707,15 @@
 lsp.Diagnostic pluginToDiagnostic(
   server.LineInfo Function(String) getLineInfo,
   plugin.AnalysisError error, {
-  @required Set<lsp.DiagnosticTag> supportedTags,
+  required Set<lsp.DiagnosticTag>? supportedTags,
 }) {
-  List<lsp.DiagnosticRelatedInformation> relatedInformation;
-  if (error.contextMessages != null && error.contextMessages.isNotEmpty) {
-    relatedInformation = error.contextMessages
+  List<lsp.DiagnosticRelatedInformation>? relatedInformation;
+  final contextMessages = error.contextMessages;
+  if (contextMessages != null && contextMessages.isNotEmpty) {
+    relatedInformation = contextMessages
         .map((message) =>
             pluginToDiagnosticRelatedInformation(getLineInfo, message))
+        .whereNotNull()
         .toList();
   }
 
@@ -746,11 +736,16 @@
   );
 }
 
-lsp.DiagnosticRelatedInformation pluginToDiagnosticRelatedInformation(
-    server.LineInfo Function(String) getLineInfo,
+lsp.DiagnosticRelatedInformation? pluginToDiagnosticRelatedInformation(
+    server.LineInfo? Function(String) getLineInfo,
     plugin.DiagnosticMessage message) {
-  var file = message.location.file;
-  var lineInfo = getLineInfo(file);
+  final file = message.location.file;
+  final lineInfo = getLineInfo(file);
+  // We shouldn't get context messages for something we can't get a LineInfo for
+  // but if we did, it's better to omit the context than fail to send the errors.
+  if (lineInfo == null) {
+    return null;
+  }
   return lsp.DiagnosticRelatedInformation(
       location: lsp.Location(
         uri: Uri.file(file).toString(),
@@ -781,8 +776,8 @@
   }
 }
 
-lsp.Location searchResultToLocation(
-    server.SearchResult result, server.LineInfo lineInfo) {
+lsp.Location? searchResultToLocation(
+    server.SearchResult result, server.LineInfo? lineInfo) {
   final location = result.location;
 
   if (lineInfo == null) {
@@ -795,7 +790,7 @@
   );
 }
 
-lsp.CompletionItemKind suggestionKindToCompletionItemKind(
+lsp.CompletionItemKind? suggestionKindToCompletionItemKind(
   Set<lsp.CompletionItemKind> supportedCompletionKinds,
   server.CompletionSuggestionKind kind,
   String label,
@@ -840,7 +835,7 @@
     }
   }
 
-  return getKindPreferences().firstWhere(isSupported, orElse: () => null);
+  return getKindPreferences().firstWhereOrNull(isSupported);
 }
 
 lsp.ClosingLabel toClosingLabel(
@@ -849,7 +844,7 @@
         range: toRange(lineInfo, label.offset, label.length),
         label: label.label);
 
-lsp.CodeActionKind toCodeActionKind(String id, lsp.CodeActionKind fallback) {
+lsp.CodeActionKind toCodeActionKind(String? id, lsp.CodeActionKind fallback) {
   if (id == null) {
     return fallback;
   }
@@ -872,9 +867,9 @@
   int replacementOffset,
   int insertLength,
   int replacementLength, {
-  @required bool includeCommitCharacters,
-  @required bool completeFunctionCalls,
-  Object resolutionData,
+  required bool includeCommitCharacters,
+  required bool completeFunctionCalls,
+  CompletionItemResolutionInfo? resolutionData,
 }) {
   // Build separate display and filter labels. Displayed labels may have additional
   // info appended (for example '(...)' on callables) that should not be included
@@ -917,9 +912,10 @@
   final supportsAsIsInsertMode =
       capabilities.completionInsertTextModes.contains(InsertTextMode.asIs);
 
-  final completionKind = suggestion.element != null
+  final element = suggestion.element;
+  final completionKind = element != null
       ? elementKindToCompletionItemKind(
-          capabilities.completionItemKinds, suggestion.element.kind)
+          capabilities.completionItemKinds, element.kind)
       : suggestionKindToCompletionItemKind(
           capabilities.completionItemKinds, suggestion.kind, label);
 
@@ -938,6 +934,7 @@
   final insertText = insertTextInfo.first;
   final insertTextFormat = insertTextInfo.last;
   final isMultilineCompletion = insertText.contains('\n');
+  final cleanedDoc = cleanDartdoc(suggestion.docComplete);
 
   // Because we potentially send thousands of these items, we should minimise
   // the generated JSON as much as possible - for example using nulls in place
@@ -954,8 +951,9 @@
     data: resolutionData,
     detail: getCompletionDetail(suggestion, completionKind,
         supportsCompletionDeprecatedFlag || supportsDeprecatedTag),
-    documentation:
-        asStringOrMarkupContent(formats, cleanDartdoc(suggestion.docComplete)),
+    documentation: cleanedDoc != null
+        ? asStringOrMarkupContent(formats, cleanedDoc)
+        : null,
     deprecated: supportsCompletionDeprecatedFlag && suggestion.isDeprecated
         ? true
         : null,
@@ -997,27 +995,28 @@
 lsp.Diagnostic toDiagnostic(
   server.ResolvedUnitResult result,
   server.AnalysisError error, {
-  @required Set<lsp.DiagnosticTag> supportedTags,
-  server.ErrorSeverity errorSeverity,
+  required Set<lsp.DiagnosticTag> supportedTags,
 }) =>
     pluginToDiagnostic((_) => result.lineInfo,
         server.newAnalysisError_fromEngine(result, error),
         supportedTags: supportedTags);
 
-lsp.Element toElement(server.LineInfo lineInfo, server.Element element) =>
-    lsp.Element(
-      range: element.location != null
-          ? toRange(lineInfo, element.location.offset, element.location.length)
-          : null,
-      name: toElementName(element),
-      kind: element.kind.name,
-      parameters: element.parameters,
-      typeParameters: element.typeParameters,
-      returnType: element.returnType,
-    );
+lsp.Element toElement(server.LineInfo lineInfo, server.Element element) {
+  final location = element.location;
+  return lsp.Element(
+    range: location != null
+        ? toRange(lineInfo, location.offset, location.length)
+        : null,
+    name: toElementName(element),
+    kind: element.kind.name,
+    parameters: element.parameters,
+    typeParameters: element.typeParameters,
+    returnType: element.returnType,
+  );
+}
 
 String toElementName(server.Element element) {
-  return element.name != null && element.name != ''
+  return element.name.isNotEmpty
       ? element.name
       : (element.kind == server.ElementKind.EXTENSION
           ? '<unnamed extension>'
@@ -1025,37 +1024,40 @@
 }
 
 lsp.FlutterOutline toFlutterOutline(
-        server.LineInfo lineInfo, server.FlutterOutline outline) =>
-    lsp.FlutterOutline(
-      kind: outline.kind.name,
-      label: outline.label,
-      className: outline.className,
-      variableName: outline.variableName,
-      attributes: outline.attributes != null
-          ? outline.attributes
-              .map(
-                  (attribute) => toFlutterOutlineAttribute(lineInfo, attribute))
-              .toList()
-          : null,
-      dartElement: outline.dartElement != null
-          ? toElement(lineInfo, outline.dartElement)
-          : null,
-      range: toRange(lineInfo, outline.offset, outline.length),
-      codeRange: toRange(lineInfo, outline.codeOffset, outline.codeLength),
-      children: outline.children != null
-          ? outline.children.map((c) => toFlutterOutline(lineInfo, c)).toList()
-          : null,
-    );
+    server.LineInfo lineInfo, server.FlutterOutline outline) {
+  final attributes = outline.attributes;
+  final dartElement = outline.dartElement;
+  final children = outline.children;
+
+  return lsp.FlutterOutline(
+    kind: outline.kind.name,
+    label: outline.label,
+    className: outline.className,
+    variableName: outline.variableName,
+    attributes: attributes != null
+        ? attributes
+            .map((attribute) => toFlutterOutlineAttribute(lineInfo, attribute))
+            .toList()
+        : null,
+    dartElement: dartElement != null ? toElement(lineInfo, dartElement) : null,
+    range: toRange(lineInfo, outline.offset, outline.length),
+    codeRange: toRange(lineInfo, outline.codeOffset, outline.codeLength),
+    children: children != null
+        ? children.map((c) => toFlutterOutline(lineInfo, c)).toList()
+        : null,
+  );
+}
 
 lsp.FlutterOutlineAttribute toFlutterOutlineAttribute(
-        server.LineInfo lineInfo, server.FlutterOutlineAttribute attribute) =>
-    lsp.FlutterOutlineAttribute(
-        name: attribute.name,
-        label: attribute.label,
-        valueRange: attribute.valueLocation != null
-            ? toRange(lineInfo, attribute.valueLocation.offset,
-                attribute.valueLocation.length)
-            : null);
+    server.LineInfo lineInfo, server.FlutterOutlineAttribute attribute) {
+  final valueLocation = attribute.valueLocation;
+  return lsp.FlutterOutlineAttribute(
+      name: attribute.name,
+      label: attribute.label,
+      valueRange: valueLocation != null
+          ? toRange(lineInfo, valueLocation.offset, valueLocation.length)
+          : null);
+}
 
 lsp.FoldingRange toFoldingRange(
     server.LineInfo lineInfo, server.FoldingRegion region) {
@@ -1068,7 +1070,7 @@
       kind: toFoldingRangeKind(region.kind));
 }
 
-lsp.FoldingRangeKind toFoldingRangeKind(server.FoldingKind kind) {
+lsp.FoldingRangeKind? toFoldingRangeKind(server.FoldingKind kind) {
   switch (kind) {
     case server.FoldingKind.COMMENT:
     case server.FoldingKind.DOCUMENTATION_COMMENT:
@@ -1123,15 +1125,17 @@
       lineInfo.getOffsetOfLine(pos.line) + pos.character);
 }
 
-lsp.Outline toOutline(server.LineInfo lineInfo, server.Outline outline) =>
-    lsp.Outline(
-      element: toElement(lineInfo, outline.element),
-      range: toRange(lineInfo, outline.offset, outline.length),
-      codeRange: toRange(lineInfo, outline.codeOffset, outline.codeLength),
-      children: outline.children != null
-          ? outline.children.map((c) => toOutline(lineInfo, c)).toList()
-          : null,
-    );
+lsp.Outline toOutline(server.LineInfo lineInfo, server.Outline outline) {
+  final children = outline.children;
+  return lsp.Outline(
+    element: toElement(lineInfo, outline.element),
+    range: toRange(lineInfo, outline.offset, outline.length),
+    codeRange: toRange(lineInfo, outline.codeOffset, outline.codeLength),
+    children: children != null
+        ? children.map((c) => toOutline(lineInfo, c)).toList()
+        : null,
+  );
+}
 
 lsp.Position toPosition(server.CharacterLocation location) {
   // LSP is zero-based, but analysis server is 1-based.
@@ -1140,8 +1144,8 @@
 }
 
 lsp.Range toRange(server.LineInfo lineInfo, int offset, int length) {
-  server.CharacterLocation start = lineInfo.getLocation(offset);
-  server.CharacterLocation end = lineInfo.getLocation(offset + length);
+  final start = lineInfo.getLocation(offset) as server.CharacterLocation;
+  final end = lineInfo.getLocation(offset + length) as server.CharacterLocation;
 
   return lsp.Range(
     start: toPosition(start),
@@ -1149,7 +1153,7 @@
   );
 }
 
-lsp.SignatureHelp toSignatureHelp(Set<lsp.MarkupKind> preferredFormats,
+lsp.SignatureHelp toSignatureHelp(Set<lsp.MarkupKind>? preferredFormats,
     server.AnalysisGetSignatureResult signature) {
   // For now, we only support returning one (though we may wish to use named
   // args. etc. to provide one for each possible "next" option when the cursor
@@ -1199,13 +1203,15 @@
     return lsp.ParameterInformation(label: getParamLabel(param));
   }
 
-  final cleanDoc = cleanDartdoc(signature.dartdoc);
+  final cleanedDoc = cleanDartdoc(signature.dartdoc);
 
   return lsp.SignatureHelp(
     signatures: [
       lsp.SignatureInformation(
         label: getSignatureLabel(signature),
-        documentation: asStringOrMarkupContent(preferredFormats, cleanDoc),
+        documentation: cleanedDoc != null
+            ? asStringOrMarkupContent(preferredFormats, cleanedDoc)
+            : null,
         parameters: signature.parameters.map(toParameterInfo).toList(),
       ),
     ],
@@ -1227,8 +1233,7 @@
     server.LineInfo lineInfo,
     server.SourceEdit edit,
     int selectionOffsetRelative,
-    int selectionLength) {
-  assert(selectionOffsetRelative != null);
+    int? selectionLength) {
   return lsp.SnippetTextEdit(
     insertTextFormat: lsp.InsertTextFormat.Snippet,
     range: toRange(lineInfo, edit.offset, edit.length),
@@ -1239,27 +1244,27 @@
 
 ErrorOr<server.SourceRange> toSourceRange(
     server.LineInfo lineInfo, Range range) {
-  if (range == null) {
-    return success(null);
-  }
-
   // If there is a range, convert to offsets because that's what
   // the tokens are computed using initially.
   final start = toOffset(lineInfo, range.start);
   final end = toOffset(lineInfo, range.end);
-  if (start?.isError ?? false) {
+  if (start.isError) {
     return failure(start);
   }
-  if (end?.isError ?? false) {
+  if (end.isError) {
     return failure(end);
   }
 
-  final startOffset = start?.result;
-  final endOffset = end?.result;
+  final startOffset = start.result;
+  final endOffset = end.result;
 
   return success(server.SourceRange(startOffset, endOffset - startOffset));
 }
 
+ErrorOr<server.SourceRange?> toSourceRangeNullable(
+        server.LineInfo lineInfo, Range? range) =>
+    range != null ? toSourceRange(lineInfo, range) : success(null);
+
 lsp.TextDocumentEdit toTextDocumentEdit(
     LspClientCapabilities capabilities, FileEditInformation edit) {
   return lsp.TextDocumentEdit(
@@ -1276,8 +1281,8 @@
   LspClientCapabilities capabilities,
   server.LineInfo lineInfo,
   server.SourceEdit edit, {
-  int selectionOffsetRelative,
-  int selectionLength,
+  int? selectionOffsetRelative,
+  int? selectionLength,
 }) {
   if (!capabilities.experimentalSnippetTextEdit ||
       selectionOffsetRelative == null) {
@@ -1348,15 +1353,6 @@
 
 lsp.MarkupContent _asMarkup(
     Set<lsp.MarkupKind> preferredFormats, String content) {
-  // It's not valid to call this function with a null format, as null formats
-  // do not support MarkupContent. [asStringOrMarkupContent] is probably the
-  // better choice.
-  assert(preferredFormats != null);
-
-  if (content == null) {
-    return null;
-  }
-
   if (preferredFormats.isEmpty) {
     preferredFormats.add(lsp.MarkupKind.Markdown);
   }
@@ -1373,16 +1369,16 @@
 }
 
 Pair<String, lsp.InsertTextFormat> _buildInsertText({
-  @required bool supportsSnippets,
-  @required bool includeCommitCharacters,
-  @required bool completeFunctionCalls,
-  @required bool isCallable,
-  @required bool isInvocation,
-  @required String defaultArgumentListString,
-  @required List<int> defaultArgumentListTextRanges,
-  @required String completion,
-  @required int selectionOffset,
-  @required int selectionLength,
+  required bool supportsSnippets,
+  required bool includeCommitCharacters,
+  required bool completeFunctionCalls,
+  required bool isCallable,
+  required bool isInvocation,
+  required String? defaultArgumentListString,
+  required List<int>? defaultArgumentListTextRanges,
+  required String completion,
+  required int selectionOffset,
+  required int selectionLength,
 }) {
   var insertText = completion;
   var insertTextFormat = lsp.InsertTextFormat.PlainText;
diff --git a/pkg/analysis_server/lib/src/lsp/notification_manager.dart b/pkg/analysis_server/lib/src/lsp/notification_manager.dart
index 0866a25..dc45460 100644
--- a/pkg/analysis_server/lib/src/lsp/notification_manager.dart
+++ b/pkg/analysis_server/lib/src/lsp/notification_manager.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/channel/lsp_channel.dart';
@@ -20,7 +18,8 @@
 
   /// The analysis server, used to fetch LineInfo in order to map plugin
   /// data structures to LSP structures.
-  LspAnalysisServer server;
+  late LspAnalysisServer
+      server; // Set externally immediately after construction
 
   LspNotificationManager(this.channel, Context pathContext)
       : super(pathContext);
@@ -31,9 +30,10 @@
       String filePath, List<protocol.AnalysisError> errors) {
     final diagnostics = errors
         .map((error) => pluginToDiagnostic(
-              server.getLineInfo,
+              // We should never errors for a file we can't get a LineInfo for
+              (path) => server.getLineInfo(path)!,
               error,
-              supportedTags: server.clientCapabilities.diagnosticTags,
+              supportedTags: server.clientCapabilities?.diagnosticTags,
             ))
         .toList();
 
diff --git a/pkg/analysis_server/lib/src/lsp/progress.dart b/pkg/analysis_server/lib/src/lsp/progress.dart
index 6b87f74..fcb2dac 100644
--- a/pkg/analysis_server/lib/src/lsp/progress.dart
+++ b/pkg/analysis_server/lib/src/lsp/progress.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'dart:async';
 import 'dart:math';
 
@@ -27,13 +25,13 @@
   ///
   /// If [token] is not supplied, a random identifier will be used.
   factory ProgressReporter.serverCreated(LspAnalysisServer server,
-          [Either2<num, String> token]) =>
+          [Either2<num, String>? token]) =>
       _ServerCreatedProgressReporter(server, token);
 
   ProgressReporter._();
 
   // TODO(dantup): Add support for cancellable progress notifications.
-  FutureOr<void> begin(String title, {String message});
+  FutureOr<void> begin(String title, {String? message});
 
   FutureOr<void> end([String message]);
 }
@@ -41,25 +39,25 @@
 class _NoopProgressReporter extends ProgressReporter {
   _NoopProgressReporter() : super._();
   @override
-  void begin(String title, {String message}) {}
+  void begin(String title, {String? message}) {}
   @override
-  void end([String message]) {}
+  void end([String? message]) {}
 }
 
 class _ServerCreatedProgressReporter extends _TokenProgressReporter {
   static final _random = Random();
-  Future<bool> _tokenBeginRequest;
+  Future<bool>? _tokenBeginRequest;
 
   _ServerCreatedProgressReporter(
     LspAnalysisServer server,
-    Either2<num, String> token,
+    Either2<num, String>? token,
   ) : super(
           server,
           token ?? Either2<num, String>.t2(_randomTokenIdentifier()),
         );
 
   @override
-  Future<void> begin(String title, {String message}) async {
+  Future<void> begin(String? title, {String? message}) async {
     assert(_tokenBeginRequest == null,
         'Begin should not be called more than once');
 
@@ -81,12 +79,13 @@
   }
 
   @override
-  Future<void> end([String message]) async {
+  Future<void> end([String? message]) async {
     // Only end the token after both create/begin have completed, and return
     // a Future to indicate that has happened to callers know when it's safe
     // to re-use the token identifier.
-    if (_tokenBeginRequest != null) {
-      final didBegin = await _tokenBeginRequest;
+    final beginRequest = _tokenBeginRequest;
+    if (beginRequest != null) {
+      final didBegin = await beginRequest;
       if (didBegin) {
         super.end(message);
       }
@@ -109,14 +108,14 @@
   _TokenProgressReporter(this._server, this._token) : super._();
 
   @override
-  void begin(String title, {String message}) {
+  void begin(String? title, {String? message}) {
     _needsEnd = true;
     _sendNotification(
         WorkDoneProgressBegin(title: title ?? 'Working…', message: message));
   }
 
   @override
-  void end([String message]) {
+  void end([String? message]) {
     if (!_needsEnd) return;
     _needsEnd = false;
     _sendNotification(WorkDoneProgressEnd(message: message));
diff --git a/pkg/analysis_server/lib/src/lsp/server_capabilities_computer.dart b/pkg/analysis_server/lib/src/lsp/server_capabilities_computer.dart
index ad0241d..65af61f 100644
--- a/pkg/analysis_server/lib/src/lsp/server_capabilities_computer.dart
+++ b/pkg/analysis_server/lib/src/lsp/server_capabilities_computer.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/client_capabilities.dart';
@@ -133,9 +131,7 @@
   ServerCapabilities computeServerCapabilities(
       LspClientCapabilities clientCapabilities) {
     final codeActionLiteralSupport = clientCapabilities.literalCodeActions;
-
     final renameOptionsSupport = clientCapabilities.renameValidation;
-
     final enableFormatter = _server.clientConfiguration.enableSdkFormatter;
     final previewCommitCharacters =
         _server.clientConfiguration.previewCommitCharacters;
@@ -204,7 +200,7 @@
       // `textDocument.codeAction.codeActionLiteralSupport`."
       codeActionProvider: dynamicRegistrations.codeActions
           ? null
-          : codeActionLiteralSupport != null
+          : codeActionLiteralSupport
               ? Either2<bool, CodeActionOptions>.t2(CodeActionOptions(
                   codeActionKinds: DartCodeActionKind.serverSupportedKinds,
                 ))
@@ -322,17 +318,17 @@
         _server.clientConfiguration.previewCommitCharacters;
 
     /// Helper for creating registrations with IDs.
-    void register(bool condition, Method method, [ToJsonable options]) {
+    void register(bool condition, Method method, [ToJsonable? options]) {
       if (condition == true) {
         registrations.add(Registration(
             id: (_lastRegistrationId++).toString(),
-            method: method.toJson(),
+            method: method.toString(),
             registerOptions: options));
       }
     }
 
     final dynamicRegistrations =
-        ClientDynamicRegistrations(_server.clientCapabilities.raw);
+        ClientDynamicRegistrations(_server.clientCapabilities!.raw);
 
     register(
       dynamicRegistrations.textSync,
@@ -524,11 +520,12 @@
         RegistrationParams(registrations: registrationsToAdd),
       );
 
-      if (registrationResponse.error != null) {
+      final error = registrationResponse.error;
+      if (error != null) {
         _server.logErrorToClient(
           'Failed to register capabilities with client: '
-          '(${registrationResponse.error.code}) '
-          '${registrationResponse.error.message}',
+          '(${error.code}) '
+          '${error.message}',
         );
       }
     }
diff --git a/pkg/analysis_server/lib/src/lsp/source_edits.dart b/pkg/analysis_server/lib/src/lsp/source_edits.dart
index f4975617..cb8fe6f 100644
--- a/pkg/analysis_server/lib/src/lsp/source_edits.dart
+++ b/pkg/analysis_server/lib/src/lsp/source_edits.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:analysis_server/lsp_protocol/protocol_generated.dart';
 import 'package:analysis_server/lsp_protocol/protocol_special.dart';
 import 'package:analysis_server/src/lsp/mapping.dart';
@@ -12,6 +10,7 @@
 import 'package:analyzer/dart/analysis/features.dart';
 import 'package:analyzer/dart/analysis/results.dart';
 import 'package:analyzer/dart/ast/token.dart';
+import 'package:analyzer/error/listener.dart';
 import 'package:analyzer/source/line_info.dart';
 import 'package:analyzer/src/dart/scanner/reader.dart';
 import 'package:analyzer/src/dart/scanner/scanner.dart';
@@ -71,17 +70,17 @@
       },
     );
     // If any change fails, immediately return the error.
-    if (result?.isError ?? false) {
+    if (result != null && result.isError) {
       return ErrorOr.error(result.error);
     }
   }
   return ErrorOr.success(Pair(newContent, serverEdits));
 }
 
-ErrorOr<List<TextEdit>> generateEditsForFormatting(
+ErrorOr<List<TextEdit>?> generateEditsForFormatting(
   ParsedUnitResult result,
   int lineLength, {
-  Range range,
+  Range? range,
 }) {
   final unformattedSource = result.content;
 
@@ -112,7 +111,8 @@
 
 List<TextEdit> _generateFullEdit(
     LineInfo lineInfo, String unformattedSource, String formattedSource) {
-  final end = lineInfo.getLocation(unformattedSource.length);
+  final end =
+      lineInfo.getLocation(unformattedSource.length) as CharacterLocation;
   return [
     TextEdit(
       range:
@@ -133,17 +133,17 @@
 ErrorOr<List<TextEdit>> _generateMinimalEdits(
   ParsedUnitResult result,
   String formatted, {
-  Range range,
+  Range? range,
 }) {
   final unformatted = result.content;
   final lineInfo = result.lineInfo;
   final rangeStart = range != null ? toOffset(lineInfo, range.start) : null;
   final rangeEnd = range != null ? toOffset(lineInfo, range.end) : null;
 
-  if (rangeStart?.isError ?? false) {
+  if (rangeStart != null && rangeStart.isError) {
     return failure(rangeStart);
   }
-  if (rangeEnd?.isError ?? false) {
+  if (rangeEnd != null && rangeEnd.isError) {
     return failure(rangeEnd);
   }
 
@@ -217,8 +217,9 @@
     // edits in the same set.
     edits.add(TextEdit(
       range: Range(
-        start: toPosition(lineInfo.getLocation(startOffset)),
-        end: toPosition(lineInfo.getLocation(endOffset)),
+        start:
+            toPosition(lineInfo.getLocation(startOffset) as CharacterLocation),
+        end: toPosition(lineInfo.getLocation(endOffset) as CharacterLocation),
       ),
       newText: newText,
     ));
@@ -276,21 +277,22 @@
 /// Iterates over a token stream returning all tokens including comments.
 Iterable<Token> _iterateAllTokens(Token token) sync* {
   while (token.type != TokenType.EOF) {
-    var commentToken = token.precedingComments;
+    Token? commentToken = token.precedingComments;
     while (commentToken != null) {
       yield commentToken;
       commentToken = commentToken.next;
     }
     yield token;
-    token = token.next;
+    token = token.next!;
   }
 }
 
 /// Parse and return the first of the given Dart source, `null` if code cannot
 /// be parsed.
-Token _parse(String s, FeatureSet featureSet) {
+Token? _parse(String s, FeatureSet featureSet) {
   try {
-    var scanner = Scanner(null, CharSequenceReader(s), null)
+    var scanner = Scanner(_SourceMock.instance, CharSequenceReader(s),
+        AnalysisErrorListener.NULL_LISTENER)
       ..configureFeatures(
         featureSetForOverriding: featureSet,
         featureSet: featureSet,
@@ -310,11 +312,18 @@
   final bool newFile;
 
   /// The selection offset, relative to the edit.
-  final int selectionOffsetRelative;
-  final int selectionLength;
+  final int? selectionOffsetRelative;
+  final int? selectionLength;
 
   FileEditInformation(this.doc, this.lineInfo, this.edits,
       {this.newFile = false,
       this.selectionOffsetRelative,
       this.selectionLength});
 }
+
+class _SourceMock implements Source {
+  static final Source instance = _SourceMock();
+
+  @override
+  dynamic noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
+}
diff --git a/pkg/analysis_server/lib/src/operation/operation_analysis.dart b/pkg/analysis_server/lib/src/operation/operation_analysis.dart
index be6d158..05538df 100644
--- a/pkg/analysis_server/lib/src/operation/operation_analysis.dart
+++ b/pkg/analysis_server/lib/src/operation/operation_analysis.dart
@@ -17,9 +17,6 @@
 Future<void> scheduleImplementedNotification(
     AnalysisServer server, Iterable<String> files) async {
   var searchEngine = server.searchEngine;
-  if (searchEngine == null) {
-    return;
-  }
   for (var file in files) {
     var unit = server.getCachedResolvedUnit(file)?.unit;
     var unitElement = unit?.declaredElement;
diff --git a/pkg/analysis_server/tool/lsp_spec/codegen_dart.dart b/pkg/analysis_server/tool/lsp_spec/codegen_dart.dart
index 68c29f7..c134d20 100644
--- a/pkg/analysis_server/tool/lsp_spec/codegen_dart.dart
+++ b/pkg/analysis_server/tool/lsp_spec/codegen_dart.dart
@@ -133,7 +133,7 @@
 }
 
 bool _isSimpleType(TypeBase type) {
-  const literals = ['num', 'String', 'bool'];
+  const literals = ['num', 'String', 'bool', 'int'];
   return type is Type && literals.contains(type.dartType);
 }
 
diff --git a/pkg/analysis_server/tool/lsp_spec/generate_all.dart b/pkg/analysis_server/tool/lsp_spec/generate_all.dart
index 1cd4b7b..81f8eb0 100644
--- a/pkg/analysis_server/tool/lsp_spec/generate_all.dart
+++ b/pkg/analysis_server/tool/lsp_spec/generate_all.dart
@@ -202,7 +202,7 @@
   }
 
   final customTypes = <AstNode>[
-    interface('DartDiagnosticServer', [field('port', type: 'number')]),
+    interface('DartDiagnosticServer', [field('port', type: 'int')]),
     interface('AnalyzerStatusParams', [field('isAnalyzing', type: 'boolean')]),
     interface('PublishClosingLabelsParams', [
       field('uri', type: 'string'),
@@ -255,7 +255,7 @@
       'CompletionItemResolutionInfo',
       [
         field('file', type: 'string'),
-        field('offset', type: 'number'),
+        field('offset', type: 'int'),
       ],
     ),
     interface(
@@ -263,11 +263,11 @@
       [
         // These fields have short-ish names because they're on the payload
         // for all suggestion-set backed completions.
-        field('libId', type: 'number'),
+        field('libId', type: 'int'),
         field('displayUri', type: 'string'),
-        field('rOffset', type: 'number'), // replacementOffset
-        field('iLength', type: 'number'), // insertLength
-        field('rLength', type: 'number'), // replacementLength
+        field('rOffset', type: 'int'), // replacementOffset
+        field('iLength', type: 'int'), // insertLength
+        field('rLength', type: 'int'), // replacementLength
       ],
       baseType: 'CompletionItemResolutionInfo',
     ),
diff --git a/pkg/analysis_server/tool/lsp_spec/typescript.dart b/pkg/analysis_server/tool/lsp_spec/typescript.dart
index 5ebb88d..7d36548 100644
--- a/pkg/analysis_server/tool/lsp_spec/typescript.dart
+++ b/pkg/analysis_server/tool/lsp_spec/typescript.dart
@@ -118,6 +118,10 @@
     'ParameterInformation': {
       'label': 'String',
     },
+    'Position': {
+      'character': 'int',
+      'line': 'int',
+    },
     'ProgressParams': {
       'value': 'object',
     },
diff --git a/pkg/analysis_server/tool/lsp_spec/typescript_parser.dart b/pkg/analysis_server/tool/lsp_spec/typescript_parser.dart
index 9dc61e0..5c9065e 100644
--- a/pkg/analysis_server/tool/lsp_spec/typescript_parser.dart
+++ b/pkg/analysis_server/tool/lsp_spec/typescript_parser.dart
@@ -46,7 +46,7 @@
   final typeName = tokenType == TokenType.STRING
       ? 'string'
       : tokenType == TokenType.NUMBER
-          ? 'number'
+          ? 'int' // all literal numeric values in LSP spec are ints
           : throw 'Unknown literal type $tokenType';
   return Type.identifier(typeName);
 }