Migration: Fix EdgeBuilder._unimplemented if node==null
Migration: Fix EdgeBuilder._unimplemented if node==null

Change-Id: Id1a297f1ee5eaf4d7111ad081a0a8c49cb334303
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/150801
Reviewed-by: 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