Remove unused import
Remove unused import

This import was accidentally included in
https://dart-review.googlesource.com/c/sdk/+/204180 but it wasn't
needed.

Change-Id: I11d70c49aba784e907bb3d2d235bedb7570489c4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/204422
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