Improvement on future: add stack trace to unhandled failures.

lib/coreimpl/future_implementation.dart

 // Copyright 2012 Google Inc. All Rights Reserved.
 // Dart core library.
 
 class FutureImpl<T> implements Future<T> {
 
   bool _isComplete = false;
 
   /**
    * Value that was provided to this Future by the Completer
    */
@@ skipping 39 matching lines @@
     final res = new FutureImpl();
     res._setValue(value);
     return res;
   }
 
   T get value {
     if (!isComplete) {
       throw new FutureNotCompleteException();
     }
     if (_exception !== null) {
-      throw _exception;
+      throw new FutureUnhandledException(_exception, stackTrace);
     }
     return _value;
   }
 
   Object get exception {
     if (!isComplete) {
       throw new FutureNotCompleteException();
     }
     return _exception;
   }
@@ skipping 12 matching lines @@
   bool get hasValue {
     return isComplete && _exception === null;
   }
 
   void then(void onSuccess(T value)) {
     if (hasValue) {
       onSuccess(value);
     } else if (!isComplete) {
       _successListeners.add(onSuccess);
     } else if (!_exceptionHandled) {
-      throw _exception;
+      throw new FutureUnhandledException(_exception, stackTrace);
     }
   }
 
   void handleException(bool onException(Object exception)) {
     if (_exceptionHandled) return;
     if (_isComplete) {
        if (_exception != null) {
          _exceptionHandled = onException(_exception);
        }
     } else {
@@ skipping 25 matching lines @@
           }
         }
       }
 
       if (hasValue) {
         for (Function listener in _successListeners) {
           listener(value);
         }
       } else {
         if (!_exceptionHandled && _successListeners.length > 0) {
-          throw _exception;
+          throw new FutureUnhandledException(_exception, stackTrace);
         }
       }
     } finally {
       for (Function listener in _completionListeners) {
         try {
           listener(this);
         } catch (e) {}
       }
     }
   }
@@ skipping 111 matching lines @@
   }
 
   void complete(T value) {
     _futureImpl._setValue(value);
   }
 
   void completeException(Object exception, [Object stackTrace]) {
     _futureImpl._setException(exception, stackTrace);
   }
 }