fix PathObserver to avoid try+catch

pkg/observe/lib/src/observable_box.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.observable_box;
+
+import 'package:observe/observe.dart';
 
 // TODO(jmesserly): should the property name be configurable?
 // That would be more convenient.
 /**
  * An observable box that holds a value. Use this if you want to store a single
  * value. For other cases, it is better to use [ObservableList],
  * [ObservableMap], or a custom [Observable] implementation based on
  * [Observable]. The property name for changes is "value".
  */
 class ObservableBox<T> extends ChangeNotifier {
   T _value;
 
   ObservableBox([T initialValue]) : _value = initialValue;
 
   @reflectable T get value => _value;
 
   @reflectable void set value(T newValue) {
     _value = notifyPropertyChange(#value, _value, newValue);
   }
 
   String toString() => '#<$runtimeType value: $value>';
 }