Cache unignorable names for IgnoreValidator.
Cache unignorable names for IgnoreValidator.

I saw from 4% to 6% analysis time used by IgnoreValidator constructor.
We did quite a few expensive operations each time, for each file.
Now we will do it once for each bucket.

Change-Id: Ia7c79b777269c9e4c10a9fb9f2864cc7249eacf9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/174230
Reviewed-by: Brian Wilkerson <1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com>
Commit-Queue: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk