Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/private_symbol_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;
 
+@MirrorsUsed(targets: "test")
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 typedef int _F(int i);
 
 class _C<_T> {
   get g {}
   set s(x) {}
   m(_p) {}
   get _g {}
@@ skipping 107 matching lines @@
   Expect.throws(() => MirrorSystem.getSymbol('_private'),
                 (e) => e is ArgumentError);
 
   var notALibraryMirror = 7;
   Expect.throws(() => MirrorSystem.getSymbol('_private', notALibraryMirror),
                 (e) => e is ArgumentError || e is TypeError);
 
   Expect.throws(() => MirrorSystem.getSymbol('public', notALibraryMirror),
                 (e) => e is ArgumentError || e is TypeError);
 }