some speculative style changes

@sethladd, two speculative style changes:

- One remove the use of cascades in the web app sample. It's a more advanced feature that's not heavily used in Dart.
- The other is to change the use of parts to libraries (for pretty much the same reason).

I think it's good to limit Dart users early exposure to more advanced, less heavily used parts of the language. Thoughts?
If that's what we wall Dart devs to do, lgtm