Report dedicated diagnostics for mixin classes that use a `with` clause
and for mixin application classes that use multiple mixins, instead of
folding both cases into `mixinClassDeclarationExtendsNotObject`.
This makes the reported error match the actual invalid construct. A
`mixin class` with a `with` clause does not have the same problem as a
`mixin class` that extends a non-`Object` superclass, and a mixin
application with multiple mixins is a separate restriction again.
Update the error verifier, diagnostic definitions, messages, and fix
status entries to use the new codes. Also guard `RemoveExtendsClause` so
it only offers a fix when an actual extends clause is present, avoiding
a bogus fix for diagnostics that are now reported on `with` clauses
instead.
Consolidate the diagnostic tests under a single mixin class declaration
test file and update expectations to use the more precise error codes.
The original impetus for this change was a crash in RemoveExtendsClause
quick fix.
Change-Id: I937276f37deb293ca1fecab3ff838ace45af2ec4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/493865
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Konstantin Shcheglov
e1404d8539