Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/delegate_library_test.dart

 // Copyright (c) 2015, 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.delegate_library;
 
+@MirrorsUsed(targets: "test.delegate_library")
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
 method(a, b, c) => "$a-$b-$c";
 methodWithNamed(a, {b:'B', c}) => "$a-$b-$c";
 methodWithOpt(a, [b, c='C']) => "$a-$b-$c";
 get getter => 'g';
 set setter(x) {
   field = x*2;
@@ skipping 23 matching lines @@
   Expect.equals('X-Y-Z', proxy.methodWithOpt('X', 'Y', 'Z'));
 
   Expect.equals('g', proxy.getter);
 
   Expect.equals(5, proxy.setter = 5);
   Expect.equals(10, proxy.field);
 
   Expect.equals(5, proxy.field = 5);
   Expect.equals(5, proxy.field);
 }