fix PathObserver to avoid try+catch

pkg/observe/lib/src/observable_list.dart

Index: pkg/observe/lib/src/observable_list.dart
diff --git a/pkg/observe/lib/src/observable_list.dart b/pkg/observe/lib/src/observable_list.dart
index 0cf181c71c48661e03926220aabd159f28763e4b..41379570f35355b203510316eca5004997c42c6c 100644
--- a/pkg/observe/lib/src/observable_list.dart
+++ b/pkg/observe/lib/src/observable_list.dart
@@ -2,7 +2,11 @@
 // 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_list;
+
+import 'dart:async';
+import 'dart:collection' show ListBase;
+import 'package:observe/observe.dart';
 
 /**
  * Represents an observable list of model values. If any items are added,