fix(generator): remove invalid sort on elements

cacheUris are sorted later.  sorting the elements can be invalid (e.g.
SimpleStringLiteral has == but no compareTo)

Closes #554
docs(contributing): add developer docs explaining how to setup and run tests

- Create a first version of developer documentation explaining how to
setup and run AngularDart tests locally and on Travis.
- Update `CONTRIBUTING.md` to refer to this new doc.

Closes #322. Assumes #545.

Closes #548