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 
You should call the method on the shadow root:
```dart
Element el = ui.shadowRoot.elementFromPoint(bounds.left.toInt() + bounds.width ~/ 2,
        bounds.top.toInt() + bounds.height ~/ 2);
```