await in frog: partial support for try-catch blocks

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";
 }
 
 /**
  * Thrown if a completer tries to set the value on
  * a future that is already complete.
  */
 class FutureAlreadyCompleteException implements Exception {
   FutureAlreadyCompleteException() {}
+  String toString() => "Exception: future already completed";
 }
 
 
 class FutureImpl<T> implements Future<T> {
 
   bool _isComplete;
 
   /**
    * Value that was provided to this Future by the Completer
    */
@@ skipping 46 matching lines @@
     return _isComplete;
   }
 
   bool get hasValue() {
     return isComplete && _exception === null;
   }
 
   void then(void onComplete(T value)) {
     if (hasValue) {
       onComplete(value);
-    } else {
+    } else if (!isComplete) {
       _listeners.add(onComplete);
+    } else if (!_exceptionHandled) {
+      throw _exception;
     }
   }
 
   void handleException(void onException(Object exception)) {
     _exceptionHandlers.add(onException);
   }
 
   void _complete() {
     _isComplete = true;
     if (_exception !== null) {
@@ skipping 46 matching lines @@
   }
 
   void complete(T value) {
     _futureImpl._setValue(value);
   }
 
   void completeException(var exception) {
     _futureImpl._setException(exception);
   }
 }