First UI test (First-start about dialog)

- Created a class to handle dialogs and simulated button presses
- Created a test to:
  - ensure that the first-run about dialog shows up.
  - press the done button.
  - ensure that the dialog disappears.

Addresses https://github.com/dart-lang/chromedeveditor/issues/2781

@dinhviethoa  @devoncarew 
@ussuri 

I tried this, but getting the element at the center-point of the modal:
```
    Rectangle<int> bounds = modalElement.getBoundingClientRect();
    Element el = document.elementFromPoint(bounds.left.toInt() + bounds.width ~/ 2,
        bounds.top.toInt() + bounds.height ~/ 2);
```

returns `spark-polymer-ui`.  Maybe this is because of the Shadow DOM.