All strong mode cleaning of dart2js.

tests/compiler/dart2js/memory_compiler.dart

 // Copyright (c) 2016, 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.
 
 library dart2js.test.memory_compiler;
 
 import 'dart:async';
 
 import 'package:compiler/compiler.dart' show DiagnosticHandler;
 import 'package:compiler/compiler_new.dart'
@@ skipping 17 matching lines @@
 export 'output_collector.dart';
 export 'package:compiler/compiler_new.dart' show CompilationResult;
 export 'diagnostic_helper.dart';
 
 class MultiDiagnostics implements CompilerDiagnostics {
   final List<CompilerDiagnostics> diagnosticsList;
 
   const MultiDiagnostics([this.diagnosticsList = const []]);
 
   @override
-  void report(Message message, Uri uri, int begin, int end, String text,
-      Diagnostic kind) {
+  void report(covariant Message message, Uri uri, int begin, int end,
+      String text, Diagnostic kind) {
     for (CompilerDiagnostics diagnostics in diagnosticsList) {
       diagnostics.report(message, uri, begin, end, text, kind);
     }
   }
 }
 
 CompilerDiagnostics createCompilerDiagnostics(
     CompilerDiagnostics diagnostics, SourceFileProvider provider,
     {bool showDiagnostics: true, bool verbose: false}) {
   CompilerDiagnostics handler = diagnostics;
@@ skipping 200 matching lines @@
       handler = (Uri uri, int begin, int end, String message, Diagnostic kind) {
         diagnosticHandler(uri, begin, end, message, kind);
         formattingHandler(uri, begin, end, message, kind);
       };
     }
   } else if (diagnosticHandler == null) {
     handler = (Uri uri, int begin, int end, String message, Diagnostic kind) {};
   }
   return handler;
 }