fix PathObserver to avoid try+catch

pkg/observe/lib/src/bind_property.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.
 
-part of observe;
+library observe.src.bind_property;
+
+import 'dart:async';
+import 'package:observe/observe.dart';
 
 /**
  * Forwards an observable property from one object to another. For example:
  *
  *     class MyModel extends Observable {
  *       StreamSubscription _sub;
  *       MyOtherModel _otherModel;
  *
  *       MyModel() {
  *         ...
@@ skipping 13 matching lines @@
   return source.changes.listen((records) {
     for (var record in records) {
       if (record is PropertyChangeRecord &&
           (record as PropertyChangeRecord).name == sourceName) {
         callback();
         break;
       }
     }
   });
 }