chore: gulpifying the tests

This PR is a proposal to use gulp to run tests, avoiding the global karma install and shortening commands.

Notes:
 - karma conf files were not touched, so it is still possible to debug tests directly in Webstorm
 - the definition of the browser to be used in Travis is kept in .travis.yml file
please rename to 'test.dart' so that the task is skipped when the Dart SDK is not available