Fix test timeouts. I forgot to include observe/mirrors_used.dart in tests from

pkg/polymer_expressions/test/syntax_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.
 
 import 'dart:async';
 import 'dart:html';
 
 import 'package:logging/logging.dart';
 import 'package:observe/observe.dart';
+import 'package:observe/mirrors_used.dart'; // make test smaller.
 import 'package:polymer_expressions/polymer_expressions.dart';
 import 'package:template_binding/template_binding.dart';
 import 'package:unittest/html_config.dart';
 import 'package:unittest/unittest.dart';
 
 main() {
   useHtmlConfiguration();
 
   group('PolymerExpressions', () {
     var testDiv;
@@ skipping 86 matching lines @@
   String getFullName() => '$_firstName $_lastName';
 
   List<String> get items => _items;
 
   void set items(List<String> value) {
     _items = notifyPropertyChange(#items, _items, value);
   }
 
   String toString() => "Person(firstName: $_firstName, lastName: $_lastName)";
 }