Use DiagnosticMessage in MockCompiler.

tests/compiler/dart2js/override_inheritance_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.
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'compiler_helper.dart';
 import 'package:compiler/src/resolution/class_members.dart'
     show MembersCreator;
 
@@ skipping 16 matching lines @@
 Future check(String source, {errors, warnings, hints, infos}) {
   return MockCompiler.create((MockCompiler compiler) {
     compiler.diagnosticHandler = createHandler(compiler, source);
     compiler.parseScript(source);
     var cls = compiler.mainApp.find('Class');
     cls.ensureResolved(compiler.resolution);
     MembersCreator.computeAllClassMembers(compiler, cls);
 
     toList(o) => o == null ? [] : o is List ? o : [o];
 
-    compareMessageKinds(source, toList(errors), compiler.errors, 'error');
+    compareMessageKinds(
+        source, toList(errors),
+        compiler.diagnosticCollector.errors, 'error');
 
-    compareMessageKinds(source, toList(warnings), compiler.warnings, 'warning');
+    compareMessageKinds(
+        source, toList(warnings),
+        compiler.diagnosticCollector.warnings, 'warning');
 
     if (infos != null) {
-      compareMessageKinds(source, toList(infos), compiler.infos, 'info');
+      compareMessageKinds(
+          source, toList(infos),
+          compiler.diagnosticCollector.infos, 'info');
     }
 
     if (hints != null) {
-      compareMessageKinds(source, toList(hints), compiler.hints, 'hint');
+      compareMessageKinds(
+          source, toList(hints),
+          compiler.diagnosticCollector.hints, 'hint');
     }
   });
 }
 
 Future testRequiredParameters() {
   return Future.wait([
     check("""
           class A {
             method() => null; // testRequiredParameters:0
           }
@@ skipping 1480 matching lines @@
           class A {
             noSuchMethod(_) => null;
             method(); // testNoSuchMethod:13
           }
           class Class extends A {
           }
           """, warnings: MessageKind.UNIMPLEMENTED_METHOD_ONE,
                infos: MessageKind.UNIMPLEMENTED_METHOD_CONT),
   ]);
 }