fixes emberjs/ember.js#12506

[Saranya-Raaj]

fixes emberjs/ember.js#12506

[rwjblue]

This seems correct to me.

@mixonic / @mmun - Any objections?

[stefanpenner]

we should be careful about perf

[chadhietala]

I second Stef's concern, get can be called a pretty significant amount of times.

[mmun]

We already lookup helpers for curlies that appear outside of attributes: https://github.com/tildeio/htmlbars/blob/master/packages/htmlbars-runtime/lib/hooks.js#L698. Doing it for attributes as well is the correct thing to do.

That said, I don't like this particular implementation. get should not be looking up helpers.
In {{foo bar}}, bar is never a helper, but with this implementation we would be checking if it was. Instead, we should add an ambiguous hook that delegates get or subexpr depending on whether the path resolves to a helper or not. The ambiguous hook would be called whenever there was a {{foo}} in the template (whether in content or in an attribute). {{foo.bar}} should not trigger the ambiguous hook however.