Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/operator_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.
 
 /// Test operators.
 library test.operator_test;
 
+@MirrorsUsed(targets: "test.operator_test")
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
 import 'stringify.dart';
 
 class Foo {
   Foo operator ~() {}
   Foo operator -() {}
 
@@ skipping 110 matching lines @@
 '>=: $FOO, '
 '>>: $FOO, '
 '[]: $FOO, '
 '[]=: $DYNAMIC, '
 '^: $FOO, '
 'unary-: $FOO, '
 '|: $FOO, '
 '~: $FOO, '
 '~/: $FOO'
 '}';