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.
Yes, that was my thought process. But now I'm thinking we should keep the tests in ember-htmlbars using the current flagging system. Then let all of the other tests in other packages use the compat system. That helps us ensure SemVer compliance, and still keeps us ready to refactor some of these test paths once 1.10 goes stable.