futures: handle exceptions immediately if the future already completed.

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() {}
   String toString() => "Exception: future has not been completed";
@@ skipping 76 matching lines @@
   void then(void onComplete(T value)) {
     if (hasValue) {
       onComplete(value);
     } else if (!isComplete) {
       _listeners.add(onComplete);
     } else if (!_exceptionHandled) {
       throw _exception;
     }
   }
 
-  void handleException(void onException(Object exception)) {
-    _exceptionHandlers.add(onException);
+  void handleException(bool onException(Object exception)) {
+    if (_exceptionHandled) return;
+    if (_isComplete) {
+       if (_exception != null) {
+         _exceptionHandled = onException(_exception);
+       }
+    } else {
+      _exceptionHandlers.add(onException);
+    }
   }
 
   void _complete() {
     _isComplete = true;
     if (_exception !== null) {
       for (Function handler in _exceptionHandlers) {
         if (handler(_exception)) {
           _exceptionHandled = true;
           break;
         }
@@ skipping 84 matching lines @@
   }
 
   void complete(T value) {
     _futureImpl._setValue(value);
   }
 
   void completeException(var exception) {
     _futureImpl._setException(exception);
   }
 }