b1594fa130
This expands the existing recovery for class headers to handle invalid, out-of-order, and duplicate clauses in extension type declaration headers. Change-Id: I7b33b91cc718e4c0673137121b8d083fa0ef1562 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/321900 Reviewed-by: Jens Johansen <jensj@google.com> Reviewed-by: Brian Wilkerson <brianwilkerson@google.com> Commit-Queue: Johnni Winther <johnniwinther@google.com>