downwards inference is pushing down dynamic

[jmesserly]

Something I've noticed: we push down dynamic as a context type in some cases, and this can break some inference because "no context" (null) is treated differently from "dynamic". It would be nice to make the code clean and consistent whatever behavior we choose. IIRC in some code review comments, @leafpetersen suggested we could let InferenceContext.setType handle this by ignoring dynamic.

We also push down dynamic | Future<dynamic> which has similar problems. That can be fixed by finding the places we create FutureUnions and make sure they don't kick in if we had a dynamic context.

related to this comment #27151 (comment)

[jmesserly]

https://codereview.chromium.org/2280023004/

[jmesserly]

I managed to clean up the bogus "flatten futures" that was happening in FutureUnion as well