fix PathObserver to avoid try+catch

pkg/observe/lib/observe.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.
 
 /**
  * Support for observing changes in model-view architectures.
  *
  * **Warning:** This library is experimental, and APIs are subject to change.
  *
  * This library is used to observe changes to [Observable] types. It also
@@ skipping 59 matching lines @@
  * *Note*: it is good practice to keep `@reflectable` annotation on
  * getters/setters so they are accessible via reflection. This will preserve
  * them from tree-shaking. You can also put this annotation on the class and it
  * preserve all of its members for reflection.
  *
  * [Tools](https://www.dartlang.org/polymer-dart/) exist to convert the first
  * form into the second form automatically, to get the best of both worlds.
  */
 library observe;
 
-import 'dart:async';
-import 'dart:collection';
-
-// Note: ObservableProperty is in this list only for the unusual use case of
-// dart2js without deploy tool. The deploy tool (see "transformer.dart") will
-// add the @reflectable annotation, which makes it work with Polymer's
-// @published.
-@MirrorsUsed(metaTargets: const [Reflectable, ObservableProperty],
-    override: 'observe')
-import 'dart:mirrors';
-
-import 'package:meta/meta.dart';
-
-// Note: this is an internal library so we can import it from tests.
-// TODO(jmesserly): ideally we could import this with a prefix, but it caused
-// strange problems on the VM when I tested out the dirty-checking example
-// above.
-import 'src/dirty_check.dart';
-
-part 'src/bind_property.dart';
-part 'src/change_notifier.dart';
-part 'src/change_record.dart';
-part 'src/compound_binding.dart';
-part 'src/list_path_observer.dart';
-part 'src/metadata.dart';
-part 'src/observable.dart';
-part 'src/observable_box.dart';
-part 'src/observable_list.dart';
-part 'src/observable_map.dart';
-part 'src/path_observer.dart';
-part 'src/to_observable.dart';
+export 'src/bind_property.dart';
+export 'src/change_notifier.dart';
+export 'src/change_record.dart';
+export 'src/compound_binding.dart';
+export 'src/list_path_observer.dart';
+export 'src/metadata.dart';
+export 'src/observable.dart' hide notifyPropertyChangeHelper, objectType;
+export 'src/observable_box.dart';
+export 'src/observable_list.dart';
+export 'src/observable_map.dart';
+export 'src/path_observer.dart';
+export 'src/to_observable.dart';