Rewrite keyword contributor tests

I did not change any of the test cases. I did change the names of the
tests to be more consistent, and in the process I found a couple of
tests where the test code was identical, so I deleted the duplicates.

I expect that there will be more cleanup that we'll want to do after
all of the contributor tests have been converted and merged together,
but I wanted this CL to be as straightforward as possible given its
size.

Change-Id: I2cbb6b5c0718fbbc36da521ee3c4fc7d00c36dcb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/297720
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>