Adjust HTTP server examples

This corresponds to [dartlang.org PR#312](https://github.com/dart-lang/dartlang.org/pull/312):
- Make consistent use of cascade operator (missing from first two examples).
- Change ‘dead-simple server’ qualifier to ‘Hello world server’, which
seems more appropriate and slightly less … coarse (note that I have not
renamed the file so as to make it easier to view the diff).
- Show how to set the content type in the Hello world server (since
this is commonly needed).
- Remove unused imports.
- Misc. cleanup.