Enable Handlebars compat mode when HTMLBars is enabled.

This is stage one in being fully backwards compat with the Ember.Handelbars API's.

Over the next few PR's I will remove all but the bare minimum required instances of the ember-htmlbars feature flag.
yeah, I'm kind of thinking this as well. I like that the helpers are tested against the internal APIs, it means when we drop the handlebars package they still Just Work.