fix PathObserver to avoid try+catch

pkg/observe/lib/src/metadata.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.metadata;
 
 /**
  * Use `@observable` to make a field automatically observable, or to indicate
  * that a property is observable.
  */
 const ObservableProperty observable = const ObservableProperty();
 
 /**
  * An annotation that is used to make a property observable.
  * Normally this is used via the [observable] constant, for example:
@@ skipping 29 matching lines @@
  *       ...
  *     }
  *     ...
  *       // This will work even if the code has been tree-shaken/minified:
  *       final monster = new Monster();
  *       new PathObserver(monster, 'health').changes.listen(...);
  */
 class Reflectable {
   const Reflectable();
 }