BREAKING CHANGE:
The Http module previously would return RxJS Observables from method calls
of the Http class. In order to support Dart, the module was refactored to
return the EventEmitter abstraction instead, which does not contain the same
combinators or subscription semantics as an RxJS Observable. However, the
EventEmitter provides a toRx() method which will return an RxJS Subject,
providing the same subscription and combinator conveniences as were
available prior to this refactor.
This is temporary, until issue #2794 is resolved, when Observables will
again be returned directly from Http class methods.
34eaf65a79