clean up old Http API

[jbdeboer]

Most of this work is fixing up apps that are using it.

[mhevery]

ping, what is this?

[jbdeboer]

https://github.com/angular/angular.dart/blob/master/lib/core_dom/http.dart#L433

[mhevery]

Work to be done:

1. remove getString() method on https://github.com/angular/angular.dart/blob/master/lib/core_dom/http.dart#L393
2. inline request method on https://github.com/angular/angular.dart/blob/master/lib/core_dom/http.dart#L646 into call method
3. make sure that everyone uses the call() or get() method for making the requets.