fix PathObserver to avoid try+catch

pkg/observe/lib/src/compound_binding.dart

Index: pkg/observe/lib/src/compound_binding.dart
diff --git a/pkg/observe/lib/src/compound_binding.dart b/pkg/observe/lib/src/compound_binding.dart
index 3d279268cf99fe3939f08a16b609ff53100c585e..86773d9992838bbff2c077cd3cc9570c28a3e95a 100644
--- a/pkg/observe/lib/src/compound_binding.dart
+++ b/pkg/observe/lib/src/compound_binding.dart
@@ -2,7 +2,10 @@
 // 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.compound_binding;
+
+import 'dart:async';
+import 'package:observe/observe.dart';
 
 /** The callback used in the [CompoundBinding.combinator] field. */
 typedef Object CompoundBindingCombinator(Map objects);
@@ -129,5 +132,5 @@ class CompoundBinding extends ChangeNotifier {
     value = null;
   }
 
-  _unobserved() => close();
+  unobserved() => close();
 }