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  
Split the tests into two kinds.  One for functionally open (checks to see if the dialog reports open) and another for visually opened (dialog not visually appearing).  The later has been implemented with a timer (probably won't implement on all dialogs due to testing duration, but useful for one test).