Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/spawn_function_root_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 lib;
+
+@MirrorsUsed(targets: "lib")
 import 'dart:mirrors';
 import 'dart:isolate';
 import 'package:expect/expect.dart';
 
 child(SendPort port) {
   LibraryMirror root = currentMirrorSystem().isolate.rootLibrary;
   Expect.isNotNull(root);
   port.send(root.uri.toString());
 }
 
 main() {
   var port;
   port = new RawReceivePort((String childRootUri) {
     LibraryMirror root = currentMirrorSystem().isolate.rootLibrary;
     Expect.isNotNull(root);
     Expect.equals(root.uri.toString(), childRootUri);
     port.close();
   });
 
   Isolate.spawn(child, port.sendPort);
 }