Use DiagnosticMessage in MockCompiler.

tests/compiler/dart2js/diagnose_ambiguous_test.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.
 
 
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/compiler_new.dart' show Diagnostic;
 import 'package:expect/expect.dart';
 import 'memory_compiler.dart';
 
 void main() {
   DiagnosticCollector collector = new DiagnosticCollector();
   asyncTest(() async {
     CompilationResult result = await runCompiler(
       memorySourceFiles: MEMORY_SOURCE_FILES,
       diagnosticHandler: collector,
       options: ['--analyze-all']);
 
     List<String> diagnostics = <String>[];
-    collector.messages.forEach((DiagnosticMessage message) {
+    collector.messages.forEach((CollectedMessage message) {
       if (message.kind == Diagnostic.VERBOSE_INFO) return;
       diagnostics.add(message.toString());
     });
     diagnostics.sort();
     var expected = [
         "memory:exporter.dart:43:47:'hest' is defined here.:info",
         "memory:library.dart:41:45:'hest' is defined here.:info",
         "memory:main.dart:0:22:'hest' is imported here.:info",
         "memory:main.dart:23:46:'hest' is imported here.:info",
         "memory:main.dart:86:92:Duplicate import of 'hest'.:warning",
@@ skipping 25 matching lines @@
 hest() {}
 """,
   'exporter.dart': """
 library exporter;
 
 export 'library.dart';
 
 hest() {}
 """,
 };