Flow analysis: expand analyzer support for "why not promoted" messages.

This CL adds support for the following scenarios to the analyzer:

- Attempt to use a non-promoted nullable expression as the iterable of
  a for-in loop

- Attempt to use a non-promoted nullable expression as the argument of
  a `yield *` statement

- Attempt to implicitly invoke `.call` on a non-promoted nullable
  expression

- Attempt to use a non-promoted nullable expression as the argument of
  a spread operator (`...`) that is not null-aware

Some of these cases are already handled by the CFE.  Others will be
addressed in a follow-up CL.

Change-Id: I3cf31b1496e1bd92fdd3f8192f04c98dff15077c
Bug: https://github.com/dart-lang/sdk/issues/44898
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/186320
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>