diff --git a/pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart b/pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart
index a0caf52..78e7fc4e 100644
--- a/pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart
@@ -284,9 +284,8 @@
       verifier.generateDuplicateShownHiddenNameHints(errorReporter);
       verifier.generateUnusedImportHints(errorReporter);
       verifier.generateUnusedShownNameHints(errorReporter);
-      // TODO(srawlins): Re-enable this check once Flutter engine path is clear.
-      // verifier.generateUnnecessaryImportHints(
-      //     errorReporter, _usedImportedElementsList);
+      verifier.generateUnnecessaryImportHints(
+          errorReporter, _usedImportedElementsList);
     }
 
     // Unused local elements.
diff --git a/pkg/analyzer/lib/src/dart/constant/constant_verifier.dart b/pkg/analyzer/lib/src/dart/constant/constant_verifier.dart
index 0e1d054..fbfbc17 100644
--- a/pkg/analyzer/lib/src/dart/constant/constant_verifier.dart
+++ b/pkg/analyzer/lib/src/dart/constant/constant_verifier.dart
@@ -469,8 +469,7 @@
         _reportErrorIfFromDeferredLibrary(
             realArgument,
             CompileTimeErrorCode
-                .INVALID_ANNOTATION_CONSTANT_VALUE_FROM_DEFERRED_LIBRARY,
-            [realArgument]);
+                .INVALID_ANNOTATION_CONSTANT_VALUE_FROM_DEFERRED_LIBRARY);
       }
     }
   }
diff --git a/pkg/analyzer/lib/src/dart/resolver/resolution_visitor.dart b/pkg/analyzer/lib/src/dart/resolver/resolution_visitor.dart
index d25fcf6..379b087 100644
--- a/pkg/analyzer/lib/src/dart/resolver/resolution_visitor.dart
+++ b/pkg/analyzer/lib/src/dart/resolver/resolution_visitor.dart
@@ -1280,7 +1280,7 @@
     // to be a DynamicTypeImpl
     Identifier name = typeName.name;
     if (!_libraryElement.shouldIgnoreUndefinedIdentifier(name)) {
-      _errorReporter.reportErrorForNode(errorCode, name, [name.name]);
+      _errorReporter.reportErrorForNode(errorCode, name);
     }
   }
 
diff --git a/pkg/analyzer/lib/src/dart/resolver/type_property_resolver.dart b/pkg/analyzer/lib/src/dart/resolver/type_property_resolver.dart
index 2a71f1d..fc870ea 100644
--- a/pkg/analyzer/lib/src/dart/resolver/type_property_resolver.dart
+++ b/pkg/analyzer/lib/src/dart/resolver/type_property_resolver.dart
@@ -112,8 +112,10 @@
       }
 
       CompileTimeErrorCode errorCode;
+      List<String> arguments;
       if (parentExpression == null) {
         errorCode = CompileTimeErrorCode.UNCHECKED_INVOCATION_OF_NULLABLE_VALUE;
+        arguments = [];
       } else {
         if (parentExpression is CascadeExpression) {
           parentExpression = parentExpression.cascadeSections.first;
@@ -121,16 +123,20 @@
         if (parentExpression is BinaryExpression) {
           errorCode = CompileTimeErrorCode
               .UNCHECKED_OPERATOR_INVOCATION_OF_NULLABLE_VALUE;
+          arguments = [name];
         } else if (parentExpression is MethodInvocation ||
             parentExpression is MethodReferenceExpression) {
           errorCode = CompileTimeErrorCode
               .UNCHECKED_METHOD_INVOCATION_OF_NULLABLE_VALUE;
+          arguments = [name];
         } else if (parentExpression is FunctionExpressionInvocation) {
           errorCode =
               CompileTimeErrorCode.UNCHECKED_INVOCATION_OF_NULLABLE_VALUE;
+          arguments = [];
         } else {
           errorCode =
               CompileTimeErrorCode.UNCHECKED_PROPERTY_ACCESS_OF_NULLABLE_VALUE;
+          arguments = [name];
         }
       }
 
@@ -150,7 +156,7 @@
       }
       _resolver.nullableDereferenceVerifier.report(
           errorCode, propertyErrorEntity, receiverType,
-          arguments: [name], messages: messages);
+          arguments: arguments, messages: messages);
       _reportedGetterError = true;
       _reportedSetterError = true;
 
diff --git a/pkg/analyzer/lib/src/error/best_practices_verifier.dart b/pkg/analyzer/lib/src/error/best_practices_verifier.dart
index eac31f6..53d62f5 100644
--- a/pkg/analyzer/lib/src/error/best_practices_verifier.dart
+++ b/pkg/analyzer/lib/src/error/best_practices_verifier.dart
@@ -1227,9 +1227,7 @@
         prefix.name, FunctionElement.LOAD_LIBRARY_NAME);
     if (loadLibraryElement != null) {
       _errorReporter.reportErrorForNode(
-          HintCode.IMPORT_DEFERRED_LIBRARY_WITH_LOAD_FUNCTION,
-          node,
-          [importedLibrary.name]);
+          HintCode.IMPORT_DEFERRED_LIBRARY_WITH_LOAD_FUNCTION, node);
       return true;
     }
     return false;
diff --git a/pkg/analyzer/lib/src/error/codes.dart b/pkg/analyzer/lib/src/error/codes.dart
index c5f54b2..2974e8f 100644
--- a/pkg/analyzer/lib/src/error/codes.dart
+++ b/pkg/analyzer/lib/src/error/codes.dart
@@ -6978,7 +6978,7 @@
   // ```
   static const CompileTimeErrorCode INVALID_MODIFIER_ON_SETTER =
       CompileTimeErrorCode('INVALID_MODIFIER_ON_SETTER',
-          "The modifier '{0}' can't be applied to the body of a setter.",
+          "Setters can't use 'async', 'async*', or 'sync*'.",
           correction: "Try removing the modifier.", hasPublishedDocs: true);
 
   /**
diff --git a/pkg/analyzer/lib/src/error/imports_verifier.dart b/pkg/analyzer/lib/src/error/imports_verifier.dart
index 4568292..878fdee 100644
--- a/pkg/analyzer/lib/src/error/imports_verifier.dart
+++ b/pkg/analyzer/lib/src/error/imports_verifier.dart
@@ -339,8 +339,7 @@
       int length = identifiers.length;
       for (int i = 0; i < length; i++) {
         Identifier identifier = identifiers[i];
-        reporter.reportErrorForNode(
-            HintCode.DUPLICATE_HIDDEN_NAME, identifier, [identifier.name]);
+        reporter.reportErrorForNode(HintCode.DUPLICATE_HIDDEN_NAME, identifier);
       }
     });
     _duplicateShownNamesMap.forEach(
@@ -348,8 +347,7 @@
       int length = identifiers.length;
       for (int i = 0; i < length; i++) {
         Identifier identifier = identifiers[i];
-        reporter.reportErrorForNode(
-            HintCode.DUPLICATE_SHOWN_NAME, identifier, [identifier.name]);
+        reporter.reportErrorForNode(HintCode.DUPLICATE_SHOWN_NAME, identifier);
       }
     });
   }
diff --git a/pkg/analyzer/lib/src/error/nullable_dereference_verifier.dart b/pkg/analyzer/lib/src/error/nullable_dereference_verifier.dart
index 35834b5..4a5d28e 100644
--- a/pkg/analyzer/lib/src/error/nullable_dereference_verifier.dart
+++ b/pkg/analyzer/lib/src/error/nullable_dereference_verifier.dart
@@ -47,6 +47,7 @@
       List<DiagnosticMessage>? messages}) {
     if (receiverType == _typeSystem.typeProvider.nullType) {
       errorCode = CompileTimeErrorCode.INVALID_USE_OF_NULL_VALUE;
+      arguments = [];
     }
     if (errorEntity is AstNode) {
       _errorReporter.reportErrorForNode(
diff --git a/pkg/analyzer/lib/src/generated/error_verifier.dart b/pkg/analyzer/lib/src/generated/error_verifier.dart
index bf8620e..73d7faf 100644
--- a/pkg/analyzer/lib/src/generated/error_verifier.dart
+++ b/pkg/analyzer/lib/src/generated/error_verifier.dart
@@ -1987,9 +1987,7 @@
       ConstructorName constructorName, TypeName typeName) {
     if (typeName.isDeferred) {
       errorReporter.reportErrorForNode(
-          CompileTimeErrorCode.CONST_DEFERRED_CLASS,
-          constructorName,
-          [typeName.name.name]);
+          CompileTimeErrorCode.CONST_DEFERRED_CLASS, constructorName);
     }
   }
 
@@ -2526,9 +2524,7 @@
     DartType type = node.typeOrThrow;
     if (type is FunctionType && type.typeFormals.isNotEmpty) {
       errorReporter.reportErrorForNode(
-          CompileTimeErrorCode.GENERIC_FUNCTION_TYPE_CANNOT_BE_BOUND,
-          node,
-          [type]);
+          CompileTimeErrorCode.GENERIC_FUNCTION_TYPE_CANNOT_BE_BOUND, node);
     }
   }
 
diff --git a/pkg/analyzer/test/source/error_processor_test.dart b/pkg/analyzer/test/source/error_processor_test.dart
index d9a581f..c9ec408 100644
--- a/pkg/analyzer/test/source/error_processor_test.dart
+++ b/pkg/analyzer/test/source/error_processor_test.dart
@@ -37,9 +37,7 @@
   ]);
 
   AnalysisError use_of_void_result = AnalysisError(
-      TestSource(), 0, 1, CompileTimeErrorCode.USE_OF_VOID_RESULT, [
-    ['x']
-  ]);
+      TestSource(), 0, 1, CompileTimeErrorCode.USE_OF_VOID_RESULT, []);
 
   // We in-line a lint code here in order to avoid adding a dependency on the
   // linter package.
diff --git a/pkg/analyzer/test/src/diagnostics/unnecessary_import_test.dart b/pkg/analyzer/test/src/diagnostics/unnecessary_import_test.dart
index a5c76d1..b6071fc 100644
--- a/pkg/analyzer/test/src/diagnostics/unnecessary_import_test.dart
+++ b/pkg/analyzer/test/src/diagnostics/unnecessary_import_test.dart
@@ -9,8 +9,7 @@
 
 main() {
   defineReflectiveSuite(() {
-    // TODO(srawlins): Re-enable this check once Flutter engine path is clear.
-    // defineReflectiveTests(UnnecessaryImportTest);
+    defineReflectiveTests(UnnecessaryImportTest);
   });
 }
 
diff --git a/pkg/analyzer/test/src/diagnostics/unnecessary_type_check_test.dart b/pkg/analyzer/test/src/diagnostics/unnecessary_type_check_test.dart
index ab55baf..e132fd9 100644
--- a/pkg/analyzer/test/src/diagnostics/unnecessary_type_check_test.dart
+++ b/pkg/analyzer/test/src/diagnostics/unnecessary_type_check_test.dart
@@ -2,7 +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.
 
-import 'package:analyzer/src/dart/error/hint_codes.dart';
 import 'package:analyzer/src/error/codes.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
diff --git a/pkg/analyzer/tool/diagnostics/diagnostics.md b/pkg/analyzer/tool/diagnostics/diagnostics.md
index e13b744..da67bc4 100644
--- a/pkg/analyzer/tool/diagnostics/diagnostics.md
+++ b/pkg/analyzer/tool/diagnostics/diagnostics.md
@@ -6713,7 +6713,7 @@
 
 ### invalid_modifier_on_setter
 
-_The modifier '{0}' can't be applied to the body of a setter._
+_Setters can't use 'async', 'async*', or 'sync*'._
 
 #### Description
 
diff --git a/pkg/dartdev/lib/dartdev.dart b/pkg/dartdev/lib/dartdev.dart
index c50d7d0..151bf0a 100644
--- a/pkg/dartdev/lib/dartdev.dart
+++ b/pkg/dartdev/lib/dartdev.dart
@@ -10,20 +10,20 @@
 import 'package:args/command_runner.dart';
 import 'package:cli_util/cli_logging.dart';
 import 'package:dart_style/src/cli/format_command.dart';
-import 'package:dartdev/src/commands/migrate.dart';
 import 'package:devtools_server/devtools_server.dart';
 import 'package:meta/meta.dart';
 import 'package:pedantic/pedantic.dart';
 import 'package:pub/pub.dart';
-
 import 'package:usage/usage.dart';
 
 import 'src/analytics.dart';
 import 'src/commands/analyze.dart';
 import 'src/commands/compile.dart';
 import 'src/commands/create.dart';
+import 'src/commands/debug_adapter.dart';
 import 'src/commands/fix.dart';
 import 'src/commands/language_server.dart';
+import 'src/commands/migrate.dart';
 import 'src/commands/run.dart';
 import 'src/commands/test.dart';
 import 'src/core.dart';
@@ -112,6 +112,7 @@
 
     addCommand(AnalyzeCommand(verbose: verbose));
     addCommand(CreateCommand(verbose: verbose));
+    addCommand(DebugAdapterCommand(verbose: verbose));
     addCommand(CompileCommand(verbose: verbose));
     addCommand(DevToolsCommand(
       verbose: verbose,
diff --git a/pkg/dartdev/lib/src/commands/debug_adapter.dart b/pkg/dartdev/lib/src/commands/debug_adapter.dart
new file mode 100644
index 0000000..e1a87b5
--- /dev/null
+++ b/pkg/dartdev/lib/src/commands/debug_adapter.dart
@@ -0,0 +1,72 @@
+// Copyright (c) 2021, 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.
+
+import 'dart:async';
+import 'dart:io';
+
+import 'package:dds/dap.dart';
+
+import '../core.dart';
+
+/// A command to start a debug adapter process that communicates over
+/// stdin/stdout using the Debug Adapter Protocol to allow editors to run debug
+/// sessions in a standard way.
+class DebugAdapterCommand extends DartdevCommand {
+  static const String cmdName = 'debug_adapter';
+
+  static const argIpv6 = 'ipv6';
+  static const argDds = 'dds';
+  static const argAuthCodes = 'auth-codes';
+  static const argTest = 'test';
+
+  DebugAdapterCommand({bool verbose = false})
+      : super(
+          cmdName,
+          'Start a debug adapter that conforms to the Debug Adapter Protocol.',
+          verbose,
+          hidden: true,
+        ) {
+    argParser
+      ..addFlag(
+        argIpv6,
+        defaultsTo: false,
+        help: 'Whether to bind DAP/VM Service/DDS to IPv6 addresses.',
+      )
+      ..addFlag(
+        argDds,
+        defaultsTo: true,
+        help: 'Whether to enable DDS for debug sessions.',
+      )
+      ..addFlag(
+        argAuthCodes,
+        defaultsTo: true,
+        help: 'Whether to enable authentication codes for VM Services.',
+      )
+      ..addFlag(
+        argTest,
+        defaultsTo: false,
+        help: 'Whether to use the "dart test" debug adapter to run tests'
+            ' and emit custom events for test progress/results.',
+      );
+  }
+
+  @override
+  FutureOr<int> run() async {
+    final args = argResults;
+    final ipv6 = args[argIpv6] as bool;
+
+    final server = DapServer(
+      stdin,
+      stdout.nonBlocking,
+      ipv6: ipv6,
+      enableDds: args[argDds],
+      enableAuthCodes: args[argAuthCodes],
+      test: args[argTest],
+    );
+
+    await server.channel.closed;
+
+    return 0;
+  }
+}
diff --git a/pkg/dartdev/pubspec.yaml b/pkg/dartdev/pubspec.yaml
index 5f509c2..fb3e6d0 100644
--- a/pkg/dartdev/pubspec.yaml
+++ b/pkg/dartdev/pubspec.yaml
@@ -17,6 +17,8 @@
   dart2native:
     path: ../dart2native
   dart_style: any
+  dds:
+    path: ../dds
   devtools_server: any
   front_end:
     path: ../front_end
diff --git a/pkg/dartdev/test/commands/debug_adapter_test.dart b/pkg/dartdev/test/commands/debug_adapter_test.dart
new file mode 100644
index 0000000..c21f476
--- /dev/null
+++ b/pkg/dartdev/test/commands/debug_adapter_test.dart
@@ -0,0 +1,29 @@
+// Copyright (c) 2021, 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.
+
+import 'package:test/test.dart';
+
+import '../utils.dart';
+
+void main() {
+  group('debug_adapter', debugAdapter, timeout: longTimeout);
+}
+
+void debugAdapter() {
+  // Implementation of debug_adapter is tested in the DDS package where the
+  // DAP implementation lives.
+  test('--help', () {
+    final p = project();
+    var result = p.runSync(['debug_adapter', '--help']);
+
+    expect(
+        result.stdout,
+        contains(
+            'Start a debug adapter that conforms to the Debug Adapter Protocol.'));
+    expect(result.stdout,
+        contains('Whether to use the "dart test" debug adapter to run tests'));
+    expect(result.stderr, isEmpty);
+    expect(result.exitCode, 0);
+  });
+}
diff --git a/pkg/dds/lib/dap.dart b/pkg/dds/lib/dap.dart
new file mode 100644
index 0000000..cb598fe
--- /dev/null
+++ b/pkg/dds/lib/dap.dart
@@ -0,0 +1,5 @@
+// Copyright (c) 2021, 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.
+
+export 'src/dap/server.dart' show DapServer;
diff --git a/pkg/dds/test/dap/integration/test_client.dart b/pkg/dds/test/dap/integration/test_client.dart
index f794ad7..0850b5c4 100644
--- a/pkg/dds/test/dap/integration/test_client.dart
+++ b/pkg/dds/test/dap/integration/test_client.dart
@@ -26,7 +26,7 @@
 
   final Logger? _logger;
   final bool captureVmServiceTraffic;
-  final _requestWarningDuration = const Duration(seconds: 5);
+  final _requestWarningDuration = const Duration(seconds: 10);
   final Map<int, _OutgoingRequest> _pendingRequests = {};
   final _eventController = StreamController<Event>.broadcast();
   int _seq = 1;
diff --git a/pkg/dds/test/dap/integration/test_server.dart b/pkg/dds/test/dap/integration/test_server.dart
index 41ea32e..ccb4c1d 100644
--- a/pkg/dds/test/dap/integration/test_server.dart
+++ b/pkg/dds/test/dap/integration/test_server.dart
@@ -12,6 +12,9 @@
 import 'package:path/path.dart' as path;
 import 'package:pedantic/pedantic.dart';
 
+/// Enable to run from local source (useful in development).
+const runFromSource = false;
+
 abstract class DapTestServer {
   Future<void> stop();
   StreamSink<List<int>> get sink;
@@ -102,13 +105,18 @@
     final ddsEntryScript =
         await Isolate.resolvePackageUri(Uri.parse('package:dds/dds.dart'));
     final ddsLibFolder = path.dirname(ddsEntryScript!.toFilePath());
-    final dapServerScript =
-        path.join(ddsLibFolder, '../tool/dap/run_server.dart');
+    final dartdevScript = path
+        .normalize(path.join(ddsLibFolder, '../../dartdev/bin/dartdev.dart'));
 
-    final _process = await Process.start(
-      Platform.resolvedExecutable,
-      [dapServerScript, 'dap', ...?additionalArgs],
-    );
+    final args = [
+      // When running from source, run the script instead of directly using
+      // the "dart debug_adapter" command.
+      if (runFromSource) dartdevScript,
+      'debug_adapter',
+      ...?additionalArgs,
+    ];
+
+    final _process = await Process.start(Platform.resolvedExecutable, args);
 
     return OutOfProcessDapTestServer._(_process, logger);
   }
diff --git a/pkg/dds/tool/dap/run_server.dart b/pkg/dds/tool/dap/run_server.dart
deleted file mode 100644
index 6130640..0000000
--- a/pkg/dds/tool/dap/run_server.dart
+++ /dev/null
@@ -1,80 +0,0 @@
-// Copyright (c) 2021, 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.
-
-import 'dart:async';
-import 'dart:io';
-
-import 'package:args/command_runner.dart';
-import 'package:dds/src/dap/server.dart';
-
-Future<void> main(List<String> arguments) async {
-  // TODO(dantup): "dap_tool" is a placeholder and will likely eventually be a
-  // "dart" command.
-  final runner = CommandRunner('dap_tool', 'Dart DAP Tool')
-    ..addCommand(DapCommand(stdin, stdout.nonBlocking));
-
-  try {
-    await runner.run(arguments);
-  } on UsageException catch (e) {
-    print(e);
-    exit(64);
-  }
-}
-
-class DapCommand extends Command {
-  static const argIpv6 = 'ipv6';
-  static const argDds = 'dds';
-  static const argAuthCodes = 'auth-codes';
-  static const argTest = 'test';
-
-  final Stream<List<int>> _inputStream;
-  final StreamSink<List<int>> _outputSink;
-
-  @override
-  final String description = 'Start a DAP debug server.';
-
-  @override
-  final String name = 'dap';
-
-  DapCommand(this._inputStream, this._outputSink) {
-    argParser
-      ..addFlag(
-        argIpv6,
-        defaultsTo: false,
-        help: 'Whether to bind DAP/VM Service/DDS to IPv6 addresses',
-      )
-      ..addFlag(
-        argDds,
-        defaultsTo: true,
-        help: 'Whether to enable DDS for debug sessions',
-      )
-      ..addFlag(
-        argAuthCodes,
-        defaultsTo: true,
-        help: 'Whether to enable authentication codes for VM Services',
-      )
-      ..addFlag(
-        argTest,
-        defaultsTo: false,
-        help: 'Whether to use the "dart test" debug adapter to run tests'
-            ' and emit custom events for test progress',
-      );
-  }
-
-  Future<void> run() async {
-    final args = argResults!;
-    final ipv6 = args[argIpv6] as bool;
-
-    final server = DapServer(
-      _inputStream,
-      _outputSink,
-      ipv6: ipv6,
-      enableDds: args[argDds],
-      enableAuthCodes: args[argAuthCodes],
-      test: args[argTest],
-    );
-
-    await server.channel.closed;
-  }
-}
diff --git a/tools/VERSION b/tools/VERSION
index 10235e0..f1f0b56 100644
--- a/tools/VERSION
+++ b/tools/VERSION
@@ -27,5 +27,5 @@
 MAJOR 2
 MINOR 15
 PATCH 0
-PRERELEASE 120
+PRERELEASE 121
 PRERELEASE_PATCH 0
\ No newline at end of file