fix up tests

test/path_observer_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 'package:observe/observe.dart';
 import 'package:unittest/unittest.dart';
 import 'package:observe/src/path_observer.dart'
     show getSegmentsOfPropertyPathForTesting,
          observerSentinelForTesting;
 
 import 'observe_test_utils.dart';
 
+import 'package:observe/mirrors_used.dart'; // make test smaller.
+import 'package:smoke/mirrors.dart';
+
 // This file contains code ported from:
 // https://github.com/rafaelw/ChangeSummary/blob/master/tests/test.js
 // Dart note: getting invalid properties is an error, unlike in JS where it
 // returns undefined. This difference comes up where we check for _throwsNSM in
 // the tests below.
 main() => dirtyCheckZone().run(() {
+  useMirrors();
+
   group('PathObserver', observePathTests);
 
   group('PropertyPath', () {
     test('toString length', () {
       expectPath(p, str, len, [keys]) {
         var path = new PropertyPath(p);
         expect(path.toString(), str);
         expect(path.length, len, reason: 'expected path length $len for $path');
         if (keys == null) {
           expect(path.isValid, isFalse);
@@ skipping 707 matching lines @@
   @observable var c;
 
   WatcherModel();
 }
 
 class Foo {
   var value;
   Foo(this.value);
   String toString() => 'Foo$value';
 }