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?
let's change `changeme` and be more prescriptive. What would a library use here?

I'm fine that this is a library and not a part, fwiw.