Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/invoke_test.dart

 // Copyright (c) 2013, 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.invoke_test;
 
+@MirrorsUsed(targets: "test.invoke_test")
 import 'dart:mirrors';
 
 import 'dart:async' show Future;
 
 import 'package:expect/expect.dart';
 
 class C {
   var field;
   C() : this.field = 'default';
   C.named(this.field);
@@ skipping 132 matching lines @@
   Expect.equals('lbar', libraryField);
   Expect.equals('lbar', lm.getField(const Symbol('libraryField')).reflectee);
   Expect.throws(() => lm.setField(const Symbol('doesntExist'), 'lbar'),
                 isNoSuchMethodError,
                 'Not defined');
 }
 
 main() {
   testSync();
 }