add initialization of _exceptionHandled in FutureImplementation.dart

corelib/src/implementation/future_implementation.dart

 // Copyright 2011 Google Inc. All Rights Reserved.
 // Dart core library.
 
 /**
  * Thrown if client tries to obtain value or exception
  * before a future has completed.
  */
 class FutureNotCompleteException implements Exception {
   FutureNotCompleteException() {}
 }
@@ skipping 32 matching lines @@
    */
   final List<Function> _listeners;
 
   /**
    * Exception handlers waiting for exceptions.
    */
   final List<Function> _exceptionHandlers;
 
   FutureImpl() : _listeners = new List(), _exceptionHandlers = new List() {
     _isComplete = false;
+    _exceptionHandled = false;
   }
 
   T get value() {
     if (!isComplete) {
       throw new FutureNotCompleteException();
     }
     if (_exception != null) {
       throw _exception;
     }
     return _value;
@@ skipping 78 matching lines @@
   }
 
   void complete(T value) {
     _futureImpl._setValue(value);
   }
 
   void completeException(var exception) {
     _futureImpl._setException(exception);
   }
 }