Don't lose the this reference for compilerHost methods.

Fixes #1545
Sounds a little ridiculous, but maybe `getNewLine` should do this too, at least for the sake of consistency.

I kind of doubt `program` methods would need it, since we're the ones producing values of the type `Program`, but it might be useful for those producing a custom parse tree. This happens to be a scenario we're seeing people want as well.