function Result() {
  this.value = undefined;
}
`finally` will be invoked regardless of the promise's fate just as native
    try/catch/finally behaves
  
    Synchronous example:
  
    ```js
    findAuthor() {
      if (Math.random() > 0.5) {
        throw new Error();
      }
      return new Author();
    }
  
    try {
      return findAuthor(); // succeed or fail
    } catch(error) {
      return findOtherAuther();
    } finally {
      // always runs
      // doesn't affect the return value
    }
    ```
  
    Asynchronous example:
  
    ```js
    findAuthor().catch(function(reason){
      return findOtherAuther();
    }).finally(function(){
      // author was either found, or not
    });
    ```
  
    @method finally
    @param {Function} callback
    @param {String} label optional string for labeling the promise.
    Useful for tooling.
    @return {Promise}