Remove EphemeralIdentifier.
Remove EphemeralIdentifier.

Change-Id: I0ae49cfcd0d4592e8951b18437f0fa3fa5d3ac5a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/259645
Reviewed-by: Samuel Rawlins <5cf2cac5eec2ad6d0eb2b0f4ff6af9a590ce2bf6@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