It may be open source, but it makes it more difficult to work with existing js libraries, and shoehorns you into a particular environment that is not standard frontend (html, css, js). Sure, Dart may compile down to js, but that adds another layer to a build process, and increases complexity unnecessarily.
As a fairly knowledgeable user of Angular & sometime contributor, this is a disturbing rumor.
The dart:js library makes it quite easy to work with existing JS libraries. Most of the teams using Dart at Google are also using some Js in their apps.
Dart encourages a very standard frontend: HTML, CSS and JavaScript, so I don't get that point. Dart compiles to JS like Haxe, TypeScript, ClojureScript, CoffeeScript, etc.
I'm not sure what "rumor" you're referring to. AngularDart is a thing and it reached beta today. That's not a rumor. AngularJS 1.2 is due out soon, and AngularJS 2.0 will be heavily influenced by the work done on AngularDart to modularize and use more web standards. That's also not a rumor.
As a fairly knowledgeable user of Angular & sometime contributor, this is a disturbing rumor.