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

pkg/template_binding/test/template_binding_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 template_binding.test.template_binding_test;
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:math' as math;
 import 'package:observe/observe.dart';
+import 'package:observe/mirrors_used.dart'; // make test smaller.
 import 'package:template_binding/template_binding.dart';
 import 'package:unittest/html_config.dart';
 import 'package:unittest/unittest.dart';
 
 // TODO(jmesserly): merge this file?
 import 'binding_syntax.dart' show syntaxTests;
 import 'utils.dart';
 
 // Note: this file ported from
 // https://github.com/Polymer/TemplateBinding/blob/fcb7a502794f19544f2d4b77c96eebb70830591d/tests/tests.js
@@ skipping 2374 matching lines @@
   }
 }
 
 class Issue18Syntax extends BindingDelegate {
   prepareBinding(path, name, node) {
     if (name != 'class') return null;
 
     return (model, _, oneTime) => new PathObserver(model, path);
   }
 }