Migration: make the element type of _ScopedLocalSet non-nullable.
Migration: make the element type of _ScopedLocalSet non-nullable.

Previously, _ScopedLocalSet extended ScopedSet<Element?>, but this was
unnecessary; it was an artifact of the fact that the migration tool
was migrated to null safety in a conservative way.  We know that all
elements of a _ScopedLocalSet are non-null elements because they all
represent actual variables declared in the user's code.

Change-Id: I4d0c86997ff3c783cee6ea84cff80363e14c3060
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/206670
Reviewed-by: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>
Commit-Queue: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@google.com>

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