Add a standalone unittest Dart app.

* Include "json_parser_test" only as a first step.
Add a standalone unittest Dart app.

* Include "json_parser_test" only as a first step.
Merge branch 'unittest-app' of https://github.com/rpaquay/chromedeveditor into unittest-app
Merge pull request #3181 from rpaquay/unittest-app

Add simple top level Dart console app for running unit tests.