feat(directives): remove the @Controller directive

Completely remove the deprecated Controller directive. Update the
demos to use setRootContext.

Pulled out of the first commit in #1269.
Ok, after hours of debugging, I think I have some understanding of how transformers work (and a few bugs to report to dart team).

expressions in @Component(templateUrl: <exp>) are resolved at transformation time starting with project root. So if I change it to 'web/bouncing_controller.html' transformer works, but now pub serve would not serve because it already start at /web.

Lib works because it gets moved to packages/angular_dart_demo which works both for transformer and pub serve.

So we should either fix the transformer before this commit, or put it in lib for now.