ListMixin map/extend generic method comments

[jmesserly]

Right now they don't match the generic method comment signatures on Iterable, so this is an error:

class _FbList<E> extends Object with ListMixin<E> implements List<E>

Marked high-pri because this is easy to hit.

CC @leafpetersen, as this is related to SDK working with strong mode.

(this is sort of area-sdk, but only affects Analyzer strong mode. Not sure best way to label it.)

[leafpetersen]

I fixed a bunch of these in the DDC copy of the sdk here: https://codereview.chromium.org/1554683002/

Lots of those SDK changes should get rolled into the main SDK.

[jmesserly]

I think @vsmenon is looking at this one.