Use DiagnosticMessage in MockCompiler.

tests/compiler/dart2js/warnings_checker.dart

 // Copyright (c) 2013, 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.
 
 // Test that dart2js produces the expected static type warnings to ensures that
 // the analyzer and dart2js agrees on the tests.
 
 import 'dart:async';
 import 'dart:io';
 import 'package:expect/expect.dart';
@@ skipping 37 matching lines @@
     // Line numbers with known unexpected warnings.
     List<int> unexpectedStatus = [];
     if (statusMap != null && statusMap.containsKey('unexpected')) {
       unexpectedStatus = statusMap['unexpected'];
     }
     // Line numbers with known missing warnings.
     List<int> missingStatus = [];
     if (statusMap != null && statusMap.containsKey('missing')) {
       missingStatus = statusMap['missing'];
     }
-    for (DiagnosticMessage message in collector.warnings) {
+    for (CollectedMessage message in collector.warnings) {
       Expect.equals(uri, message.uri);
       int lineNo = file.getLine(message.begin);
       if (expectedWarnings.containsKey(lineNo)) {
         unseenWarnings.remove(lineNo);
       } else if (!unexpectedStatus.contains(lineNo+1)) {
         warningsMismatch = true;
         print(file.getLocationMessage(
             'Unexpected warning: ${message.message}',
             message.begin, message.end));
       }
     }
     if (!unseenWarnings.isEmpty) {
       for (int lineNo in unseenWarnings) {
         if (!missingStatus.contains(lineNo+1)) {
           warningsMismatch = true;
           String line = expectedWarnings[lineNo];
           print('$uri [${lineNo+1}]: Missing static type warning.');
           print(line);
         }
       }
     }
   }).then((_) {
     Expect.isFalse(warningsMismatch);
   }));
 }