Move PathObserver to mdv_observe package

pkg/mdv_observe/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:html' show PathObserver;
-import 'package:unittest/html_config.dart';
 import 'package:mdv_observe/mdv_observe.dart';
 import 'package:unittest/unittest.dart';
 
 // This file contains code ported from:
 // https://github.com/rafaelw/ChangeSummary/blob/master/tests/test.js
 
 main() {
-  useHtmlConfiguration();
   group('PathObserver', observePathTests);
 }
 
 observePath(obj, path) => new PathObserver(obj, path);
 
 sym(x) => new Symbol(x);
 
 toSymbolMap(Map map) {
   var result = new ObservableMap.linked();
   map.forEach((key, value) {
@@ skipping 221 matching lines @@
   }
   void setValueWorkaround(key, newValue) {
     if (key == fieldName) value = newValue;
   }
 
   toString() => '#<$runtimeType $fieldName: $_value>';
 }
 
 _record(key, oldValue, newValue, [kind = ChangeRecord.FIELD]) =>
     new ChangeRecord(key, oldValue, newValue, kind: kind);