Migration: Remove _ScopedLocalSet class.
Migration: Remove _ScopedLocalSet class.

This class wasn't really necessary; it was just a container for a few
extra methods used only by the EdgeBuilder.  By moving those methods
to the EdgeBuilder and removing the _ScopedLocalSet class, we open up
the possibility of expanding the capabilities of those methods to take
advantage of other information the EdgeBuilder has access to.  (I will
do this in follow-up CLs).

No functional change.

Change-Id: I7cf19b8cf81db0413f4a9e747d46fb619d5eac4e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/206840
Reviewed-by: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>
Commit-Queue: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@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