Use DiagnosticMessage in MockCompiler.

tests/compiler/dart2js/compiler_helper.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.
 
 library compiler_helper;
 
 import 'dart:async';
 import "package:expect/expect.dart";
 
 import 'package:compiler/src/elements/elements.dart'
@@ skipping 130 matching lines @@
   OutputCollector outputCollector = new OutputCollector();
   MockCompiler compiler = compilerFor(
       code, uri, coreSource: coreSource, disableInlining: disableInlining,
       minify: minify, expectedErrors: expectedErrors,
       trustTypeAnnotations: trustTypeAnnotations,
       expectedWarnings: expectedWarnings,
       outputProvider: outputCollector);
   compiler.diagnosticHandler = createHandler(compiler, code);
   return compiler.run(uri).then((compilationSucceded) {
     Expect.isTrue(compilationSucceded,
-                   'Unexpected compilation error(s): ${compiler.errors}');
+                  'Unexpected compilation error(s): '
+                  '${compiler.diagnosticCollector.errors}');
     return outputCollector.getOutput('', 'js');
   });
 }
 
 Future compileAndCheck(String code,
                        String name,
                        check(MockCompiler compiler, lego.Element element),
                        {int expectedErrors, int expectedWarnings}) {
   Uri uri = new Uri(scheme: 'source');
   MockCompiler compiler = compilerFor(code, uri,
@@ skipping 118 matching lines @@
     regexp = regexp.replaceAll(xRe, '(?:$anyIdentifier)');
     final spaceRe = new RegExp('\\s+');
     regexp = regexp.replaceAll(spaceRe, '(?:\\s*)');
     if (shouldMatch) {
       Expect.isTrue(new RegExp(regexp).hasMatch(generated));
     } else {
       Expect.isFalse(new RegExp(regexp).hasMatch(generated));
     }
   });
 }