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 
Perhaps it should be part of a Dialog's contract that it provides an `onClose` or `onHidden` event. The dialog would then know how to deal with all the timing issues, like if there's an animation and appropriately listening for animation-end events.