anchor.diff:      // Continuations are copied where they are bound, before processing
       // expressions in the scope of their binding.
       push(node.handler);
  -    Continuation handler = _copies[node.handler] =
  -        new Continuation(node.handler.parameters.map(_definitions.copy)
  -            .toList());
  +    Continuation handler = _copies[node.handler] = new Continuation(
  +        node.handler.parameters.map(_definitions.copy).toList());
       plug(new LetHandler(handler, null));
       return node.body;
     }
  
positive.code: 
    Definition visitForeignCode(ForeignCode node) {
      return new ForeignCode(node.codeTemplate, node.storedType,
          getList(node.argumentRefs),
          node.nativeBehavior,
          dependency: node.dependency);
    }
  }
  
negative.code:   ///
    ///     if (condition) value.selectorName();
    ///
    ReceiverCheck.generalCheck(Primitive value, Selector selector,
        SourceInformation sourceInformation, Primitive condition)
        : this(value, selector, sourceInformation,
              condition: condition, useSelector: true, isNullCheck: false);
  
    bool get isSafeForElimination => false;
    bool get isSafeForReordering => false;
  