Use DiagnosticMessage in MockCompiler.

tests/compiler/dart2js/private_test.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.
 
 import 'dart:async';
 
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/diagnostics/messages.dart' show MessageKind;
 import 'package:compiler/src/io/source_file.dart';
 
@@ skipping 54 matching lines @@
                     import '$PRIVATE_SOURCE_URI';
   
                     void main() {
                       PublicClass publicClass;
                       $text
                     }
                     ''';
     Uri uri = Uri.parse('src:public');
     compiler.registerSource(uri, source);
     return compiler.run(uri).then((_) {
-      compareWarningKinds(text, expectedWarnings, compiler.warnings);
+      DiagnosticCollector collector = compiler.diagnosticCollector;
+      compareWarningKinds(text, expectedWarnings, collector.warnings);
     });
   };
 }
 
 void main() {
   asyncTest(() => Future.forEach([
     // Read from private variable.
     analyze('var value = _privateVariable;', MessageKind.CANNOT_RESOLVE),
     // Write to private variable.
     analyze('_privateVariable = 0;', MessageKind.CANNOT_RESOLVE),
@@ skipping 78 matching lines @@
     analyze('var value = publicClass.publicGetter;'),
     // Call public setter on public class.
     analyze('publicClass.publicSetter = 0;'),
     // Access public method on public class.
     analyze('var value = publicClass.publicMethod;'),
     // Call public method on public class.
     analyze('publicClass.publicMethod();'),
   ], (f) => f()));
 }