Improve the Dart metamodel to match the Dart specification

Removed the concept of Application, renamed fields and methods to
variables and functions among other changes.

Bug-Url: https://github.com/dartdesigner/Dart-Designer/issues/31
Change-Id: Idcbd45b832557bbbdfe968efc60c5c258e068647
Add support default values for fields

Bug-Url: https://github.com/dartdesigner/Dart-Designer/issues/25
Change-Id: Ifbfb28afd8ed789033898d7aecb0fcd673920f52