Create a metamodel of the Dart language

Bug-Url: https://github.com/dartdesigner/Dart-Designer/issues/16
Change-Id: Idaa0d5015f63d0d0690f5f6070e326cea155fbb8
Switch to Tycho 0.20.0 and Eclipse Luna

Bug-Url: https://github.com/dartdesigner/Dart-Designer/issues/19
Change-Id: I3a0690a66ccf9f403d1cf910156eb014de6b72a8
Fix an issue with the message for the tracking.

Change-Id: I260adf8b6d9d094e50a5af690aca1c48dbac6c7c
Add the first version of the Dart designers:

- Add a Package Diagram
- Add an Explorer Diagram

Bug-Url: https://github.com/dartdesigner/Dart-Designer/issues/17
Bug-Url: https://github.com/dartdesigner/Dart-Designer/issues/18
Change-Id: Ic1576f646ff451a53cca27486db724ac0924a4ea