Update all non-tutorial references to dart.js.

Staged at:
http://dartjs.dart-lang.appspot.com/

Especially see:
http://dartjs.dart-lang.appspot.com/articles/embedding-in-html/#dartjs-script
http://dartjs.dart-lang.appspot.com/samples/sunflower/sunflower.html
http://dartjs.dart-lang.appspot.com/samples/spirodraw/app/spirodraw.html

Review URL: https://chromiumcodereview.appspot.com/12026031