Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/invoke_throws_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_throws_test;
 
+@MirrorsUsed(targets: "test.invoke_throws_test")
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
 class MyException {
 }
 
 class Class {
   Class.noException();
   Class.generative() { throw new MyException(); }
@@ skipping 44 matching lines @@
                 (e) => e is MyException);
 
   LibraryMirror lm = reflectClass(Class).owner;
   Expect.throws(() => lm.getField(#libraryGetter),
                 (e) => e is MyException);
   Expect.throws(() => lm.setField(#librarySetter, ['arg']),
                 (e) => e is MyException);
   Expect.throws(() => lm.invoke(#libraryFunction, []),
                 (e) => e is MyException);
 }