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

@devoncarew 
I'm not sure that modalElement.opened means that it's showing.
I would check style.opacity / style.display and maybe position/size on the screen?