Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/parameter_is_const_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.parameter_is_const;
 
+@MirrorsUsed(targets: "test.parameter_is_const")
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
 class Class {
   foo(
   const /// 01: compile-time error
   param) {}
 }
 
 main() {
   MethodMirror mm = reflectClass(Class).declarations[#foo];
   Expect.isFalse(mm.parameters.single.isConst);
 }