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 
I think it would be more reliable to replace the `display` and `visibility` tests with `document.elementFromPoint()`: that would also account for the element possibly having 0 size, or being overlapped by another element, etc.