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?
Disagree with this change. Method cascades are a fantastic feature of Dart. It shows off a unique feature, a difference from other common solutions.

If we go back to traditional method calls, I'm afraid this snippet doesn't give me any reason to use Dart.

We want our starter projects to be the best, opinionated, and prescriptive ways to build web apps with Dart. Certainly that implies we recommend and promote using our useful language features like cascades.