Don't clear library context when nothing to do.
Don't clear library context when nothing to do.

This is the first step, the next step is to keep some libraries
even when there is a change, but the changed file does not affect
these libraries.

Change-Id: I3b1388be7733f3725ca632b4a67909393d893915
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/206442
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