All strong mode cleaning of dart2js.

tests/compiler/dart2js/unparser2_test.dart

 // Copyright (c) 2012, 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:expect/expect.dart";
 import "package:compiler/src/parser/element_listener.dart";
 import "package:compiler/src/parser/node_listener.dart";
 import "package:front_end/src/fasta/parser.dart";
 import "package:front_end/src/fasta/scanner.dart";
 import "package:compiler/src/tree/tree.dart";
@@ skipping 84 matching lines @@
   Node node = listener.popNode();
   Expect.isTrue(listener.nodes.isEmpty);
   return unparse(node);
 }
 
 class MessageCollector extends DiagnosticReporter {
   List<String> messages;
   MessageCollector() {
     messages = [];
   }
-  void internalError(node, String reason) {
+  void internalError(node, covariant String reason) {
     messages.add(reason);
     throw reason;
   }
 
   void log(message) {
     messages.add(message);
   }
 
   noSuchMethod(Invocation invocation) => throw 'unsupported operation';
 }