Remove unused fields from Reference.
Remove unused fields from Reference.

Change-Id: I0e7703f48321ccc868f8f389c8bb83f373adc33c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/208026
Reviewed-by: Phil Quitslund <3faecbdd7b7be83f493f037bf1d597e6bb9d0848@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