Migrate NonErrorResolverTest to null safety

In order to do this, I split the test into two:

* many test cases were concerned with type promotion
  which only seemed to pertain to pre-null safe code.
* some code could not be made valid in both null safe
  and pre-null safe environments.

Bug: dart-lang/sdk#44666
Change-Id: I818a39f2e3eb3cae17de18b008d9e4c7b8ff84c5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/184844
Reviewed-by: Konstantin Shcheglov <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>

Author: srawlins