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.  I would prefer to setup an animation-end event handler with a timeout, but that might be outside the scope of ui-tests.  Maybe I could just poll every 200 ms and timeout after 5000 or something.