Add `nonNulls`, `indexed` and other extensions on iterables.

Also adds `firstOrNull`, `lastOrNull`, `singleOrNull`, and `elementAtOrNull`.

I chose `nonNulls` instead of `whereNotNull()`.
The latter should have been a getter, but isn't.
Making it a getter in the platform libraries would be confusing,
and make migrating from `package:collection`'s function harder,
so instead I gave it a completely new name.
The alternative would be to retain the non-idiomatic `whereNotNull()`
for familiarity. But then it would be tool late to fix.



Fixes #49928

BUG= https://github.com/dart-lang/sdk/issues/49928

CoreLibraryReviewExempt: Everybody's on vacation, everybody everywhere.
Change-Id: If464e3bd6bc97cbeefc3e5084b4cbaadac3f1e95
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/290760
Reviewed-by: Aske Simon Christensen <askesc@google.com>
Reviewed-by: Nate Bosch <nbosch@google.com>
Commit-Queue: Lasse Nielsen <lrn@google.com>