fix PathObserver to avoid try+catch

pkg/observe/lib/src/bind_property.dart

Index: pkg/observe/lib/src/bind_property.dart
diff --git a/pkg/observe/lib/src/bind_property.dart b/pkg/observe/lib/src/bind_property.dart
index a31d1ab747c5875fc39825656416109e404673db..59f867f7dc94603fdb35434ec1c1b535c9c45225 100644
--- a/pkg/observe/lib/src/bind_property.dart
+++ b/pkg/observe/lib/src/bind_property.dart
@@ -2,7 +2,10 @@
 // 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: