Use @MirrorsUsed to prevent analysis server tests from slowing down in dart2js.

pkg/analysis_server/test/mocks.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.
 
 library mocks;
 
 import 'dart:async';
 import 'dart:io';
+
+@MirrorsUsed(targets: 'mocks', override: '*')
 import 'dart:mirrors';
 
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analysis_server/src/analysis_logger.dart';
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/channel.dart';
 import 'package:analysis_server/src/protocol.dart';
 import 'package:matcher/matcher.dart';
 import 'package:mock/mock.dart';
@@ skipping 171 matching lines @@
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 }
 
 /**
  * A mock [Source].  Particularly useful when all that is needed is the
  * encoding.
  */
 class MockSource extends Mock implements Source {
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 }