All strong mode cleaning of dart2js.

tests/compiler/dart2js/diagnostic_helper.dart

 // Copyright (c) 2015, 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.diagnostic_helper;
 
 import 'dart:collection';
 
 import 'package:compiler/compiler_new.dart'
     show CompilerDiagnostics, Diagnostic;
@@ skipping 17 matching lines @@
   String toString() {
     return '${message != null ? message.kind : ''}'
         ':$uri:$begin:$end:$text:$kind';
   }
 }
 
 class DiagnosticCollector implements CompilerDiagnostics {
   List<CollectedMessage> messages = <CollectedMessage>[];
 
   @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) {
     messages.add(new CollectedMessage(message, uri, begin, end, text, kind));
   }
 
   Iterable<CollectedMessage> filterMessagesByKinds(List<Diagnostic> kinds) {
     return messages
         .where((CollectedMessage message) => kinds.contains(message.kind));
   }
 
   Iterable<CollectedMessage> get errors {
     return filterMessagesByKinds([Diagnostic.ERROR]);
@@ skipping 128 matching lines @@
       String expectedValue = '${arguments[key]}';
       String foundValue = '${message.arguments[key]}';
       if (expectedValue != foundValue) {
         return 'Expected argument $key with value $expectedValue, '
             'found $foundValue.';
       }
     }
     return null;
   };
 }