Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/libraries_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.libraries_test;
 
+@MirrorsUsed(targets: "test.libraries_test")
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
 main() {
   MirrorSystem mirrors = currentMirrorSystem();
   Expect.isNotNull(mirrors, 'mirrors is null');
 
   Map<Uri, LibraryMirror> libraries = mirrors.libraries;
   Expect.isNotNull(libraries, 'libraries is null');
 
   Expect.isTrue(libraries.isNotEmpty);
   LibraryMirror mirrorsLibrary = libraries[Uri.parse('dart:mirrors')];
   if (mirrorsLibrary == null) {
     // In minified mode we don't preserve the URIs.
     mirrorsLibrary = libraries.values
         .firstWhere((LibraryMirror lm) => lm.simpleName == #dart.mirrors);
     Uri uri = mirrorsLibrary.uri;
     Expect.equals("https", uri.scheme);
     Expect.equals("dartlang.org", uri.host);
     Expect.equals("/dart2js-stripped-uri", uri.path);
   }
 
   ClassMirror cls = mirrorsLibrary.declarations[#LibraryMirror];
   Expect.isNotNull(cls, 'cls is null');
 
   Expect.equals(#dart.mirrors.LibraryMirror, cls.qualifiedName);
   Expect.equals(reflectClass(LibraryMirror), cls);
 }