diff --git a/pkg/analysis_server/lib/plugin/analysis/occurrences/occurrences_core.dart b/pkg/analysis_server/lib/plugin/analysis/occurrences/occurrences_core.dart
index afc2f1a..7483aa4 100644
--- a/pkg/analysis_server/lib/plugin/analysis/occurrences/occurrences_core.dart
+++ b/pkg/analysis_server/lib/plugin/analysis/occurrences/occurrences_core.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:analyzer_plugin/protocol/protocol_common.dart' show Occurrences;
 
 /// An object used to record occurrences into.
diff --git a/pkg/analysis_server/lib/plugin/edit/assist/assist_core.dart b/pkg/analysis_server/lib/plugin/edit/assist/assist_core.dart
index 113cbcf..c4469ea 100644
--- a/pkg/analysis_server/lib/plugin/edit/assist/assist_core.dart
+++ b/pkg/analysis_server/lib/plugin/edit/assist/assist_core.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:analyzer_plugin/protocol/protocol_common.dart'
     show SourceChange;
 import 'package:analyzer_plugin/utilities/assist/assist.dart';
diff --git a/pkg/analysis_server/lib/plugin/edit/assist/assist_dart.dart b/pkg/analysis_server/lib/plugin/edit/assist/assist_dart.dart
index b8a9946..3154306 100644
--- a/pkg/analysis_server/lib/plugin/edit/assist/assist_dart.dart
+++ b/pkg/analysis_server/lib/plugin/edit/assist/assist_dart.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:analyzer/dart/analysis/results.dart';
 import 'package:analyzer/instrumentation/service.dart';
 import 'package:analyzer_plugin/utilities/change_builder/change_workspace.dart';
diff --git a/pkg/analysis_server/lib/plugin/edit/fix/fix_core.dart b/pkg/analysis_server/lib/plugin/edit/fix/fix_core.dart
index db3b1b8..f3873dc 100644
--- a/pkg/analysis_server/lib/plugin/edit/fix/fix_core.dart
+++ b/pkg/analysis_server/lib/plugin/edit/fix/fix_core.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:analyzer/error/error.dart';
 import 'package:analyzer_plugin/protocol/protocol_common.dart'
     show SourceChange;
diff --git a/pkg/analysis_server/lib/src/plugin/plugin_locator.dart b/pkg/analysis_server/lib/src/plugin/plugin_locator.dart
index 58f8aa2..4453aea 100644
--- a/pkg/analysis_server/lib/src/plugin/plugin_locator.dart
+++ b/pkg/analysis_server/lib/src/plugin/plugin_locator.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:analyzer/file_system/file_system.dart';
 
 /// An object used to locate a plugin within a package.
@@ -26,7 +24,7 @@
   /// The resource provider used to access the file system.
   final ResourceProvider resourceProvider;
 
-  final Map<String, String> pluginMap = <String, String>{};
+  final Map<String, String?> pluginMap = {};
 
   /// Initialize a newly created plugin locator to use the given
   /// [resourceProvider] to access the file system.
@@ -48,12 +46,12 @@
   ///
   /// This method does not validate the content of the plugin directory before
   /// returning it.
-  String findPlugin(String packageRoot) {
+  String? findPlugin(String packageRoot) {
     return pluginMap.putIfAbsent(packageRoot, () => _findPlugin(packageRoot));
   }
 
   /// The implementation of [findPlugin].
-  String _findPlugin(String packageRoot) {
+  String? _findPlugin(String packageRoot) {
     var packageFolder = resourceProvider.getFolder(packageRoot);
     // TODO(brianwilkerson) Re-enable this after deciding how we want to deal
     // with discovery of plugins.
diff --git a/pkg/analysis_server/lib/src/plugin/request_converter.dart b/pkg/analysis_server/lib/src/plugin/request_converter.dart
index 0909ca1..559d7ee 100644
--- a/pkg/analysis_server/lib/src/plugin/request_converter.dart
+++ b/pkg/analysis_server/lib/src/plugin/request_converter.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/protocol/protocol_generated.dart' as server;
 import 'package:analyzer_plugin/protocol/protocol_generated.dart' as plugin;
 
@@ -24,10 +22,10 @@
       server.AnalysisSetSubscriptionsParams params) {
     var serverSubscriptions = params.subscriptions;
     var pluginSubscriptions = <plugin.AnalysisService, List<String>>{};
-    for (var service in serverSubscriptions.keys) {
+    for (var entry in serverSubscriptions.entries) {
+      var service = entry.key;
       try {
-        pluginSubscriptions[convertAnalysisService(service)] =
-            serverSubscriptions[service];
+        pluginSubscriptions[convertAnalysisService(service)] = entry.value;
       } catch (exception) {
         // Ignore the exception. It indicates that the service isn't one that
         // should be passed along to plugins.
diff --git a/pkg/analysis_server/lib/src/plugin/result_collector.dart b/pkg/analysis_server/lib/src/plugin/result_collector.dart
index a016ed1..7d695f3 100644
--- a/pkg/analysis_server/lib/src/plugin/result_collector.dart
+++ b/pkg/analysis_server/lib/src/plugin/result_collector.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
-
 /// A function used to determine whether results should be collected for the
 /// file with the given [path].
 typedef ShouldCollectPredicate = bool Function(String path);
@@ -16,7 +14,7 @@
 
   /// A function used to determine whether results should be collected for the
   /// file whose path is passed in as an argument.
-  final ShouldCollectPredicate _shouldCollect;
+  final ShouldCollectPredicate? _shouldCollect;
 
   /// A multi-keyed map, where the first key is the (normalized and absolute)
   /// path to the file associated with the results, and the second is the id of
@@ -25,7 +23,7 @@
   final Map<String, Map<String, E>> resultMap = <String, Map<String, E>>{};
 
   /// Initialize a newly created result manager.
-  ResultCollector(this.serverId, {ShouldCollectPredicate predicate})
+  ResultCollector(this.serverId, {ShouldCollectPredicate? predicate})
       : _shouldCollect = predicate;
 
   /// Clear any results that have been contributed for the file with the given
@@ -65,8 +63,9 @@
   /// Return `true` if this collector is collecting results associated with the
   /// given [filePath].
   bool isCollectingFor(String filePath) {
-    if (_shouldCollect != null) {
-      return _shouldCollect(filePath);
+    var predicate = _shouldCollect;
+    if (predicate != null) {
+      return predicate(filePath);
     }
     return resultMap.containsKey(filePath);
   }
@@ -76,7 +75,8 @@
   void putResults(String filePath, String pluginId, E partialResults) {
     var fileResults = resultMap[filePath];
     if (fileResults == null) {
-      if (_shouldCollect != null && _shouldCollect(filePath)) {
+      var predicate = _shouldCollect;
+      if (predicate != null && predicate(filePath)) {
         resultMap[filePath] = <String, E>{pluginId: partialResults};
       }
     } else {
diff --git a/pkg/analysis_server/lib/src/plugin/result_converter.dart b/pkg/analysis_server/lib/src/plugin/result_converter.dart
index a4cd788..3a0983b 100644
--- a/pkg/analysis_server/lib/src/plugin/result_converter.dart
+++ b/pkg/analysis_server/lib/src/plugin/result_converter.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/protocol/protocol_generated.dart' as server;
 import 'package:analysis_server/src/protocol/protocol_internal.dart' as server;
 import 'package:analyzer_plugin/protocol/protocol_common.dart';
diff --git a/pkg/analysis_server/lib/src/plugin/result_merger.dart b/pkg/analysis_server/lib/src/plugin/result_merger.dart
index 5194674..be59bb0 100644
--- a/pkg/analysis_server/lib/src/plugin/result_merger.dart
+++ b/pkg/analysis_server/lib/src/plugin/result_merger.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:collection';
 
 import 'package:analysis_server/protocol/protocol_generated.dart';
@@ -209,7 +207,7 @@
   /// the plugins. If a plugin contributes a navigation region that overlaps a
   /// region from a previous plugin, the overlapping region will be omitted.
   /// (For these purposes, nested regions are considered to be overlapping.)
-  AnalysisNavigationParams mergeNavigation(
+  AnalysisNavigationParams? mergeNavigation(
       List<AnalysisNavigationParams> partialResultList) {
     var count = partialResultList.length;
     if (count == 0) {
@@ -284,7 +282,7 @@
       for (var j = 0; j < targets.length; j++) {
         var target = targets[j];
         var newIndex = fileMap[target.fileIndex];
-        if (target.fileIndex != newIndex) {
+        if (newIndex != null && target.fileIndex != newIndex) {
           target = NavigationTarget(target.kind, newIndex, target.offset,
               target.length, target.startLine, target.startColumn,
               codeOffset: target.codeOffset, codeLength: target.codeLength);
@@ -303,7 +301,8 @@
         var region = regions[j];
         var newTargets = region.targets
             .map((int oldTarget) => targetMap[oldTarget])
-            .toList();
+            .toList()
+            .cast<int>();
         if (region.targets != newTargets) {
           region = NavigationRegion(region.offset, region.length, newTargets);
         }
@@ -429,23 +428,27 @@
     /// Merge the children of the [newOutline] into the list of children of the
     /// [mergedOutline].
     void mergeChildren(Outline mergedOutline, Outline newOutline) {
-      for (var newChild in newOutline.children) {
-        var mergedChild = outlineMap[computeKey(newChild.element)];
-        if (mergedChild == null) {
-          // The [newChild] isn't in the existing list.
-          var copiedOutline = copyMap.putIfAbsent(
-              mergedOutline,
-              () => Outline(
-                  mergedOutline.element,
-                  mergedOutline.offset,
-                  mergedOutline.length,
-                  mergedOutline.codeOffset,
-                  mergedOutline.codeLength,
-                  children: mergedOutline.children.toList()));
-          copiedOutline.children.add(newChild);
-          addToMap(newChild);
-        } else {
-          mergeChildren(mergedChild, newChild);
+      var newChildren = newOutline.children;
+      if (newChildren != null) {
+        for (var newChild in newChildren) {
+          var mergedChild = outlineMap[computeKey(newChild.element)];
+          if (mergedChild == null) {
+            // The [newChild] isn't in the existing list.
+            var mergedChildren = _copyList(mergedOutline.children);
+            mergedChildren.add(newChild);
+            copyMap.putIfAbsent(
+                mergedOutline,
+                () => Outline(
+                    mergedOutline.element,
+                    mergedOutline.offset,
+                    mergedOutline.length,
+                    mergedOutline.codeOffset,
+                    mergedOutline.codeLength,
+                    children: mergedChildren));
+            addToMap(newChild);
+          } else {
+            mergeChildren(mergedChild, newChild);
+          }
         }
       }
     }
@@ -533,7 +536,7 @@
   ///
   /// The feedbacks in the [partialResultList] are expected to all be of the
   /// same type. If that expectation is violated, and exception might be thrown.
-  RefactoringFeedback mergeRefactoringFeedbacks(
+  RefactoringFeedback? mergeRefactoringFeedbacks(
       List<RefactoringFeedback> feedbacks) {
     var count = feedbacks.length;
     if (count == 0) {
@@ -549,24 +552,24 @@
       // The feedbacks are empty, so there's nothing to merge.
       return first;
     } else if (first is ExtractLocalVariableFeedback) {
-      var coveringExpressionOffsets = first.coveringExpressionOffsets == null
-          ? <int>[]
-          : first.coveringExpressionOffsets.toList();
-      var coveringExpressionLengths = first.coveringExpressionLengths == null
-          ? <int>[]
-          : first.coveringExpressionLengths.toList();
+      var coveringExpressionOffsets =
+          _copyList(first.coveringExpressionOffsets);
+      var coveringExpressionLengths =
+          _copyList(first.coveringExpressionLengths);
       var names = first.names.toList();
       var offsets = first.offsets.toList();
       var lengths = first.lengths.toList();
       for (var i = 1; i < count; i++) {
-        ExtractLocalVariableFeedback feedback = feedbacks[i];
+        var feedback = feedbacks[i] as ExtractLocalVariableFeedback;
         // TODO(brianwilkerson) This doesn't ensure that the covering data is in
         // the right order and consistent.
-        if (feedback.coveringExpressionOffsets != null) {
-          coveringExpressionOffsets.addAll(feedback.coveringExpressionOffsets);
+        var coveringOffsets = feedback.coveringExpressionOffsets;
+        if (coveringOffsets != null) {
+          coveringExpressionOffsets.addAll(coveringOffsets);
         }
-        if (feedback.coveringExpressionLengths != null) {
-          coveringExpressionLengths.addAll(feedback.coveringExpressionLengths);
+        var coveringLengths = feedback.coveringExpressionLengths;
+        if (coveringLengths != null) {
+          coveringExpressionLengths.addAll(coveringLengths);
         }
         for (var name in feedback.names) {
           if (!names.contains(name)) {
@@ -593,7 +596,7 @@
       var offsets = first.offsets.toList();
       var lengths = first.lengths.toList();
       for (var i = 1; i < count; i++) {
-        ExtractMethodFeedback feedback = feedbacks[i];
+        var feedback = feedbacks[i] as ExtractMethodFeedback;
         if (returnType.isEmpty) {
           returnType = feedback.returnType;
         }
@@ -667,7 +670,7 @@
   /// The returned result will contain the concatenation of the potential edits.
   /// If two or more plugins produce the same potential edit, then the resulting
   /// list of potential edits will contain duplications.
-  EditGetRefactoringResult mergeRefactorings(
+  EditGetRefactoringResult? mergeRefactorings(
       List<EditGetRefactoringResult> partialResultList) {
     /// Return the result of merging the given list of source [changes] into a
     /// single source change.
@@ -679,7 +682,7 @@
     /// edits will be merged at the level of the file being edited, but will be
     /// a concatenation of the individual edits within each file, even if
     /// multiple plugins contribute duplicate or conflicting edits.
-    SourceChange mergeChanges(List<SourceChange> changes) {
+    SourceChange? mergeChanges(List<SourceChange> changes) {
       var count = changes.length;
       if (count == 0) {
         return null;
@@ -711,7 +714,6 @@
           }
         }
         linkedEditGroups.addAll(change.linkedEditGroups);
-        message ??= change.message;
         selection ??= change.selection;
       }
       return SourceChange(message,
@@ -731,26 +733,33 @@
     var optionsProblems = result.optionsProblems.toList();
     var finalProblems = result.finalProblems.toList();
     var feedbacks = <RefactoringFeedback>[];
-    if (result.feedback != null) {
-      feedbacks.add(result.feedback);
+    var feedback = result.feedback;
+    if (feedback != null) {
+      feedbacks.add(feedback);
     }
     var changes = <SourceChange>[];
-    if (result.change != null) {
-      changes.add(result.change);
+    var change = result.change;
+    if (change != null) {
+      changes.add(change);
     }
-    var potentialEdits = result.potentialEdits.toList();
+    var potentialEdits = _copyList(result.potentialEdits);
     for (var i = 1; i < count; i++) {
       var result = partialResultList[1];
       initialProblems.addAll(result.initialProblems);
       optionsProblems.addAll(result.optionsProblems);
       finalProblems.addAll(result.finalProblems);
-      if (result.feedback != null) {
-        feedbacks.add(result.feedback);
+      var feedback = result.feedback;
+      if (feedback != null) {
+        feedbacks.add(feedback);
       }
-      if (result.change != null) {
-        changes.add(result.change);
+      var change = result.change;
+      if (change != null) {
+        changes.add(change);
       }
-      potentialEdits.addAll(result.potentialEdits);
+      var edits = result.potentialEdits;
+      if (edits != null) {
+        potentialEdits.addAll(edits);
+      }
     }
     return EditGetRefactoringResult(
         initialProblems, optionsProblems, finalProblems,
@@ -797,4 +806,7 @@
     return !((leftStart <= rightStart && rightEnd <= leftEnd) ||
         (rightStart <= leftStart && leftEnd <= rightEnd));
   }
+
+  /// Return a copy of the [list], or an empty list if [list] is `null`.
+  List<E> _copyList<E>(List<E>? list) => list == null ? <E>[] : list.toList();
 }
diff --git a/pkg/analysis_server/lib/src/services/correction/assist.dart b/pkg/analysis_server/lib/src/services/correction/assist.dart
index 6b01848..2929cc2 100644
--- a/pkg/analysis_server/lib/src/services/correction/assist.dart
+++ b/pkg/analysis_server/lib/src/services/correction/assist.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/plugin/edit/assist/assist_dart.dart';
 import 'package:analyzer/dart/analysis/results.dart';
 import 'package:analyzer/instrumentation/service.dart';
diff --git a/pkg/analysis_server/test/src/plugin/plugin_locator_test.dart b/pkg/analysis_server/test/src/plugin/plugin_locator_test.dart
index 5b71ce6..cf45244 100644
--- a/pkg/analysis_server/test/src/plugin/plugin_locator_test.dart
+++ b/pkg/analysis_server/test/src/plugin/plugin_locator_test.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/src/plugin/plugin_locator.dart';
 import 'package:analyzer/src/test_utilities/resource_provider_mixin.dart';
 import 'package:test/test.dart';
@@ -17,10 +15,10 @@
 
 @reflectiveTest
 class PluginLocatorTest with ResourceProviderMixin {
-  String packageRoot;
-  String pubspecPath;
-  String defaultDirPath;
-  PluginLocator locator;
+  late String packageRoot;
+  late String pubspecPath;
+  late String defaultDirPath;
+  late PluginLocator locator;
 
   void setUp() {
     packageRoot = newFolder('/package').path;
diff --git a/pkg/analysis_server/test/src/plugin/request_converter_test.dart b/pkg/analysis_server/test/src/plugin/request_converter_test.dart
index 42125bb..7ca3d2a 100644
--- a/pkg/analysis_server/test/src/plugin/request_converter_test.dart
+++ b/pkg/analysis_server/test/src/plugin/request_converter_test.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/protocol/protocol_generated.dart' as server;
 import 'package:analysis_server/src/plugin/request_converter.dart';
 import 'package:analyzer_plugin/protocol/protocol_common.dart';
@@ -62,7 +60,7 @@
   }
 
   void test_convertAnalysisUpdateContentParams() {
-    var serverFiles = <String, dynamic>{
+    var serverFiles = <String, Object>{
       'file1': AddContentOverlay('content1'),
       'file2': AddContentOverlay('content2'),
     };
diff --git a/pkg/analysis_server/test/src/plugin/result_collector_test.dart b/pkg/analysis_server/test/src/plugin/result_collector_test.dart
index a8892b5..39a4486 100644
--- a/pkg/analysis_server/test/src/plugin/result_collector_test.dart
+++ b/pkg/analysis_server/test/src/plugin/result_collector_test.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/src/plugin/result_collector.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
diff --git a/pkg/analysis_server/test/src/plugin/result_converter_test.dart b/pkg/analysis_server/test/src/plugin/result_converter_test.dart
index 7eaaa6e..220ca9a 100644
--- a/pkg/analysis_server/test/src/plugin/result_converter_test.dart
+++ b/pkg/analysis_server/test/src/plugin/result_converter_test.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/protocol/protocol_generated.dart' as server;
 import 'package:analysis_server/src/plugin/result_converter.dart';
 import 'package:analyzer_plugin/protocol/protocol_common.dart';
diff --git a/pkg/analysis_server/test/src/plugin/result_merger_test.dart b/pkg/analysis_server/test/src/plugin/result_merger_test.dart
index 0116e27..934e335 100644
--- a/pkg/analysis_server/test/src/plugin/result_merger_test.dart
+++ b/pkg/analysis_server/test/src/plugin/result_merger_test.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/protocol/protocol_generated.dart';
 import 'package:analysis_server/src/plugin/result_merger.dart';
 import 'package:analyzer_plugin/protocol/protocol_common.dart';
diff --git a/pkg/analysis_server/test/stress/utilities/server.dart b/pkg/analysis_server/test/stress/utilities/server.dart
index 81d3c23..888056a 100644
--- a/pkg/analysis_server/test/stress/utilities/server.dart
+++ b/pkg/analysis_server/test/stress/utilities/server.dart
@@ -307,7 +307,7 @@
 
   /// Remove any existing overlays.
   void removeAllOverlays() {
-    Map<String, dynamic> files = HashMap<String, dynamic>();
+    var files = <String, Object>{};
     for (var path in filesWithOverlays) {
       files[path] = RemoveContentOverlay();
     }
@@ -370,8 +370,8 @@
     _send('analysis.setSubscriptions', params);
   }
 
-  void sendAnalysisUpdateContent(Map<String, dynamic> files) {
-    files.forEach((String path, dynamic overlay) {
+  void sendAnalysisUpdateContent(Map<String, Object> files) {
+    files.forEach((path, overlay) {
       if (overlay is AddContentOverlay) {
         filesWithOverlays.add(path);
       } else if (overlay is RemoveContentOverlay) {
diff --git a/tools/VERSION b/tools/VERSION
index 77ebaab..f94b5ca 100644
--- a/tools/VERSION
+++ b/tools/VERSION
@@ -27,5 +27,5 @@
 MAJOR 2
 MINOR 13
 PATCH 0
-PRERELEASE 198
+PRERELEASE 199
 PRERELEASE_PATCH 0
\ No newline at end of file