Support watching sub-expressions (JSON values) as is done for function parameters

[caitp]

Cross-referencing angular/angular.js#7663

Basically, we do this already for function paramters, it needs to happen for JSON values too.

In fact, if a JSON literal expression contains non-const expressions, then most likely we should exclusively be watching the subexpressions, and not the JSON object itself --- so that we don't end up replacing the whole object and creating garbage.

We already do this for PureFunctionAST (and I don't think this should be removed as cargo cult), so we ought to do it for JSON literal properties too. I can't recall if we're already doing that, but I don't believe we have tests for it. I'll check

[caitp]

This isn't exclusively about "Bind-once", there are other benefits to doing this. I know people keep saying things like "Bind-once won't be necessary in the future because of Object.observe", but frankly this is not true, and I can't keep re-iterating the reasons why it's not true, it makes my throat sore. So there are benefits for non-bindonce-usecases, and benefits for bind-once too.