MigrationResolutionHooks._fixBuilder is not nullable
MigrationResolutionHooks._fixBuilder is not nullable

Change-Id: I23e6c51002624028f6c1f075b20ca831895279f6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/253741
Reviewed-by: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@google.com>
Auto-Submit: Samuel Rawlins <5cf2cac5eec2ad6d0eb2b0f4ff6af9a590ce2bf6@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