Since there were already non-negative tests with the same numbers, I
gave them meaningful names.
Merged a couple of tests into the same file when it made sense since
multitests let us do that.
Deleted prefix1_negative_test, because it essentially tested only that
a random identifier does not resolve, which is not particularly related
to prefixes.
Change-Id: I91718f6df5126f123d9a8ea3abbc6898797db2db
Reviewed-on: https://dart-review.googlesource.com/52985
Reviewed-by: Sigmund Cherem <sigmund@google.com>
