fix PathObserver to avoid try+catch

pkg/observe/lib/src/observable_list.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_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,
  * removed, or replaced, then observers that are listening to [changes]
  * will be notified.
  */
 class ObservableList<E> extends ListBase<E> with ChangeNotifier {
   List<ListChangeRecord> _listRecords;
 
   /** The inner [List<E>] with the actual storage. */
@@ skipping 255 matching lines @@
 
       if (added > 0 || removed > 0) {
         notifyChange(new ListChangeRecord(startIndex, addedCount: added,
             removedCount: removed));
       }
 
       offset += removed - added;
     }
   }
 }