map<O>  method 
Implementation
UITestChainNode<U, O, T> map<O>(O Function(E e) mapper) =>
    UITestChainNode(testChainRoot, mapper(element), this as T);UITestChainNode<U, O, T> map<O>(O Function(E e) mapper) =>
    UITestChainNode(testChainRoot, mapper(element), this as T);