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.
Ok, so this is definitely the major push here. So it seems you thing the HTMLBars helpers should be tested through the handlebars compat layer compiler?

Because that will be the only public template compilation endpoint in 1.10?