Implemented dropdownToggle in dart

Some time ago I implemented it for my own pet project and now discovering your attempt to port the whole angular.ui package to dart I wanted to contribute.

As you can see I also added 2 files **_specs.dart** and **jasmine_syntax.dart**. I found them from angular.dart-s team own repository and I think it makes porting a lot easier, because fewer modifications are required to port tests (the syntax is basically the same).

Hope you accept and maybe we can collaborate on porting other directives as well.