As far as I know the release will be next year. But you can use angular 1.4 with angular 2 in the same application. This means you can prepare for the next release and move one part at a time to angular 2. I recommend angular 1.4 with TypeScript.
@Jan Vladimir Mostert, it seems to be for Angular 2.0. Regarding the code-base, I think you could develop your solution In "vanilla" Js or TypeScript without having to install anything but Angular ( v1 or v2). However, Dart might need complementary plugin to work since Angular needs clear Javascript to work. But that's just my opinion ^^.
@Edouard, that's very interesting! Does that apply to Angular1 or Angular2?
If Angular2, does that means AngularJS 2, Angular Dart 2 and Angular TypeScript will be the same code-base, only with different bindings?
Are you recommending mixing Dart with TypeScript's AngularJS 1.4 ?
Angular Dart seems to have stopped at 1.0.0, so Angular Dart doesn't seem like an option at the moment.