Use DiagnosticMessage in MockCompiler.

tests/compiler/dart2js/show_package_warnings_test.dart

 // Copyright (c) 2014, 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 the '--show-package-warnings' option works as intended.
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/commandline_options.dart';
@@ skipping 57 matching lines @@
   checkUriSchemes(collector.warnings);
   Expect.equals(hints, collector.hints.length,
                 'Unexpected hints: ${collector.hints}');
   checkUriSchemes(collector.hints);
   Expect.equals(infos, collector.infos.length,
                 'Unexpected infos: ${collector.infos}');
   checkUriSchemes(collector.infos);
   print('==================================================================');
 }
 
-void checkUriSchemes(Iterable<DiagnosticMessage> messages) {
-  for (DiagnosticMessage message in messages) {
+void checkUriSchemes(Iterable<CollectedMessage> messages) {
+  for (CollectedMessage message in messages) {
     if (message.uri != null) {
       Expect.notEquals('package', message.uri.scheme,
           "Unexpected package uri `${message.uri}` in message: $message");
     }
   }
 }
 
 void main() {
   asyncTest(() async {
     await test(
@@ skipping 21 matching lines @@
         showPackageWarnings: true,
         // From package:pkg_error1 and package:pkg_error2:
         warnings: 2, hints: 2, infos: 2);
     await test(
         Uri.parse('package:pkg_noerror/pkg_noerror.dart'),
         showPackageWarnings: false,
         hints: 2 /* from summary */);
   });
 }