anchor.diff:      _isComplete = true;
       if (_exception !== null) {
         for (Function handler in _exceptionHandlers) {
  -        if (handler(_exception)) {
  +        // Explicitly check for true here so that if the handler returns null,
  +        // we don't get an exception in checked mode.
  +        if (handler(_exception) == true) {
             _exceptionHandled = true;
             break;
           }
         }
       }
  +
       if (hasValue) {
         for (Function listener in _listeners) {
           listener(value);
  
positive.code: // Copyright 2011 Google Inc. All Rights Reserved.
  // Dart core library.
  
  class FutureImpl<T> implements Future<T> {
  
    bool _isComplete;
  
    /**
     * Value that was provided to this Future by the Completer
  
negative.code:    */
    final List<Function> _exceptionHandlers;
  
    FutureImpl()
      : _listeners = [],
        _exceptionHandlers = [];
  
    factory FutureImpl.immediate(T value) {
      final res = new FutureImpl();
  