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

pkg/polymer_expressions/test/eval_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 eval_test;
 
 import 'dart:async';
 
 // Import mirrors to cause all mirrors to be retained by dart2js.
 // The tests reflect on LinkedHashMap.length and String.length.
 import 'dart:mirrors';
 
 import 'package:polymer_expressions/eval.dart';
 import 'package:polymer_expressions/filter.dart';
 import 'package:polymer_expressions/parser.dart';
 import 'package:unittest/unittest.dart';
 import 'package:observe/observe.dart';
+import 'package:observe/mirrors_used.dart'; // make test smaller.
 
 main() {
   reflectClass(Object); // suppress unused import warning
 
   group('eval', () {
     test('should return the model for an empty expression', () {
       expectEval('', 'model', 'model');
     });
 
     test('should handle the "this" keyword', () {
@@ skipping 400 matching lines @@
   return Future.wait([future, new Future(() {
     expect(passed, true, reason: "Didn't receive a change notification on $s");
   })]);
 }
 
 // Regression test from https://code.google.com/p/dart/issues/detail?id=13459
 class WordElement extends Observable {
   @observable List chars1 = 'abcdefg'.split('');
   @reflectable List filteredList(List original) => [original[0], original[1]];
 }