199feac353
State more explicitly that sealed classes are abstract and lead the user to a better fix with the error message. Bug: https://github.com/dart-lang/sdk/issues/52073 Change-Id: Id24c6cb187ee5497ca2819f930c48ff5aa8d07fc Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/296025 Reviewed-by: Brian Wilkerson <brianwilkerson@google.com> Reviewed-by: Paul Berry <paulberry@google.com> Commit-Queue: Kallen Tu <kallentu@google.com>