a26a140d88
Since some of the async transformers return `null`, signaling that they remove nodes, they must extend the RemovingTransformer in order to migrate to null safety where arbitrary node removal is not possible. Change-Id: I8e20e2c61ce23699fe32c7450aacdd15467da5f5 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/191923 Reviewed-by: Aske Simon Christensen <askesc@google.com>