AngularJS and AngularDart are two separate projects in two different languages, sharing the same philosophy. This requires two of Google's developer teams to maintain two codebases. Angular 2.0 is still in development and its release date is unknown, yet there are some fundamental concepts known already.
Angular 2.0 is developed in AtScript, which is a language created by Google that extends Microsoft's TypeScript. TypeScript itself is a superset of JavaScript and should be partly compatible with the ECMAScript 6 standard in the future. Right now, it has to be compiled to JavaScript as well.
That said, there will be just one codebase for Angular 2.0 in AtScript, which can be compiled into JavaScript or Dart.
That's an obvious question. Angular 2.0 has to be easy to use for both Dart and JavaScript developers, and here's the problem with Dart. Dart doesn't compile into code that is easily reusable in JavaScript. In Chapter 1, Getting Started with Dart, and Chapter 2, Practical Dart, we already talked about how the Dart team is canceling efforts to implement the Dart VM into Chrome, and they're focusing on improving the dart2js compiler instead (or maybe switching to a completely new compiler https://github.com/dart-lang/dev_compiler), which is a logical step in this situation.