Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/parameter_of_mixin_app_constructor_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.
 
 library test.parameter_of_mixin_app_constructor;
 
+@MirrorsUsed(targets: "test.parameter_of_mixin_app_constructor")
 import 'dart:mirrors';
 import 'stringify.dart';
 
 class MapView {
   final _map;
   MapView(map) : this._map = map;
 }
 
 abstract class UnmodifiableMapMixin {
   someFunctionality() {}
@@ skipping 68 matching lines @@
          ' final, type = Type(s(dynamic), top-level))]',
          soleConstructorOf(mp.superclass.superclass).parameters);
   expect('[Parameter(s(p1) in s(S),'
          ' type = Class(s(int) in s(dart.core), top-level)),'
          ' Parameter(s(p2) in s(S),'
          ' type = Class(s(String) in s(dart.core), top-level))]',
          soleConstructorOf(mp.superclass.superclass.superclass).parameters);
   expect('[]',
          soleConstructorOf(mp.superclass.superclass.superclass.superclass).parameters);
 }