Update ListMixin/ListBase documentation and `insert` for Null Safety.
Update ListMixin/ListBase documentation and `insert` for Null Safety.

Document that `add` must be implemented when using
`ListMixin` or `ListBase` with Null Safety.

Changes ListMixin.insert to not grow the list itself,
but instead rely on `add` to do that.
This means that a user of ListMixin/ListBase
*only* needs to implement `add`
to support growing a list with non-nullable elements
(which they do need to because of Null Safety).

Fixes #42496

Bug: http://dartbug.com/42496
Change-Id: I9476f3111330d7c7f1878f165e066f86d3108b87
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/152853
Commit-Queue: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>
Reviewed-by: Nate Bosch <dc212c74d8afde9c690fdd09e74d3e5c5d5a3de0@google.com>
Reviewed-by: Nicholas Shahan <03847abd2d8aadfe859ecb98095a31f9f515dc47@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk