anchor.diff:  /// Tools, such as the analyzer, can provide feedback if
   ///
   /// * the annotation is associated with a declaration other than a public
  -///   instance member in a class, or
  +///   instance member in a class or mixin, or
   /// * the member is referenced outside of the defining library.
   const _VisibleForOverriding visibleForOverriding =
       const _VisibleForOverriding();
  
positive.code: /// Tools, such as the analyzer, can provide feedback if
  ///
  /// * the annotation is associated with a declaration not in the `lib` folder
  ///   of a package, or
  /// * the declaration is referenced outside of its the defining library or a
  ///   library which is in the `test` folder of the defining package.
  const _VisibleForTesting visibleForTesting = const _VisibleForTesting();
  
negative.code: ///   without invoking the overridden method.
  const _MustCallSuper mustCallSuper = const _MustCallSuper();
  
  /// Used to annotate a class, mixin, or extension declaration `C`. Indicates
  /// that any type arguments declared on `C` are to be treated as optional.
  /// Tools such as the analyzer and linter can use this information to suppress
  /// warnings that would otherwise require type arguments on `C` to be provided.
  const _OptionalTypeArgs optionalTypeArgs = const _OptionalTypeArgs();
  
  /// Used to annotate an instance member (method, getter, setter, operator, or
  