fixes #403, generating a link to a CSS file even if it wasn't generated

Required some refactoring to the order we emit things:
* entry points are processed last
* MainPageEmitter was emitting the entry point as well as doing random HTML rewrites. Now it only does the main dart code. Renamed to EntryPointEmitter.
* Most HTML changes happen in transformMainHtml
* fixed handling of schemes other than HTTP
* removed duplicate code path for rel=components

Review URL: https://chromiumcodereview.appspot.com//12849003