Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/invoke_private_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_private_test;
 
+@MirrorsUsed(targets: "invoke_private_test")
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
 class C {
   var _field;
   C() : this._field = 'default';
   C._named(this._field);
 
   get _getter => 'get $_field';
@@ skipping 59 matching lines @@
   result = lm.getField(#_libraryGetter);
   Expect.equals('lget a priori', result.reflectee);
   result = lm.getField(#_libraryField);
   Expect.equals('a priori', result.reflectee);
 
   lm.setField(#_librarySetter, 'lfoo');
   Expect.equals('lset lfoo', _libraryField);
   lm.setField(#_libraryField, 'lbar');
   Expect.equals('lbar', _libraryField);
 }