Dart -- without being in Chrome -- has the same userbase as CoffeeScript, ClojureScript, or any other language that also compiles to JavaScript.
There are some real potential wins with being in Chrome, many of which are political, but there are no technical reasons for not being in Chrome to block Dart's adoption.
Except Dart isn't trying to be another "compiles to javascript" language like CS, Dart is trying to completely replace js which is why it's so much more difficult to implement. They have to build an entire VM to rival V8.
