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 
If `#topUi` is up and running by the time this object is created, I'd cache it instead of extracting via `querySelector` every time it's used, since it can potentially be used a lot.