move new dart-js interop article back to its old URL

staged at:
https://js-dot-dart-lang.appspot.com/articles/dart-js/
https://js-dot-dart-lang.appspot.com/articles/js-dart-interop/

I'm afraid we might have lost all the search juice of the original article, but in case we haven't... I proposing moving the new one to the old one's URL.

BUG= https://code.google.com/p/dart/issues/detail?id=15073
R=sethladd@google.com

Review URL: https://chromiumcodereview.appspot.com/86853002