Helpers depending on helpers

[Rich-Harris]

Re #479, but is a broader issue — would be good if helpers didn't need to be entirely self-contained, because then they could share common functionality. For example this line in dispatchObservers should really be replaced with a call to differs. It would also mean that methods with dev mode variants could do their checks then just call the underlying function.

We could do this in an automatic way by parsing ASTs, analysing them to discover references to identifiers that aren't parameters or built-ins, marking those as dependencies, then (if necessary) rewriting those identifiers to their aliases.

(We only need to do this when the helpers are inlined, of course.)