fix PathObserver to avoid try+catch

pkg/observe/lib/src/compound_binding.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.compound_binding;
+
+import 'dart:async';
+import 'package:observe/observe.dart';
 
 /** The callback used in the [CompoundBinding.combinator] field. */
 typedef Object CompoundBindingCombinator(Map objects);
 
 /**
  * CompoundBinding is an object which knows how to listen to multiple path
  * values (registered via [bind]) and invoke its [combinator] when one or more
  * of the values have changed and set its [value] property to the return value
  * of the function. When any value has changed, all current values are provided
  * to the [combinator] in the single `values` argument.
@@ skipping 106 matching lines @@
    */
   void close() {
     for (var binding in _observers.values) {
       binding.cancel();
     }
     _observers.clear();
     _values.clear();
     value = null;
   }
 
-  _unobserved() => close();
+  unobserved() => close();
 }