Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/to_string_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.to_string_test;
 
+@MirrorsUsed(targets: "test.to_string_test")
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
 expect(expected, actual) => Expect.stringEquals(expected, '$actual');
 
 class Foo {
   var field;
   method() {}
 }
 
 main() {
   var mirrors = currentMirrorSystem();
   expect("TypeMirror on 'dynamic'", mirrors.dynamicType);
   expect("TypeMirror on 'void'", mirrors.voidType);
   expect("LibraryMirror on 'test.to_string_test'",
          mirrors.findLibrary(#test.to_string_test));
   expect("InstanceMirror on 1", reflect(1));
   expect("ClassMirror on 'Foo'", reflectClass(Foo));
   expect("VariableMirror on 'field'",
          reflectClass(Foo).declarations[#field]);
   expect("MethodMirror on 'method'",
          reflectClass(Foo).declarations[#method]);
   String s = reflect(main).toString();
   Expect.isTrue(s.startsWith("ClosureMirror on '"), s);
 }