Extension types don't override but rather redeclare so we should probably discourage/disallow the use of the @override annotation on extension type members and provide an alternative annotation such as @redeclare to communicate when an extension type member is intended to be redeclaring. (Along with that we should consider a lint, like unnecessary_redeclare to flag misplaced annotations (e.g., like unnecesssary_overrides).

/cc @dart-lang/language-team @scheglov @bwilkerson @jacob314