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 
Oh, but Dialog already has such a contract, now that I come to think of it: it's the `opened` property, which you can monitor using a `PathObserver`. There is an additional advantage to using that, because `paper-dialog` (if we ever use it internally) also has only `opened`, but no explicit `onClose` or something.