anchor.diff:    @override
     Source get librarySource => library.source;
   
  +  @override
  +  get session => mixin.session;
  +
     /// Returns true iff this class was declared using the syntax
     /// `class B = A with M;`, i.e., if it is an explicitly named mixin
     /// application.
  
positive.code: /// [assetId]. This function returns null if we cannot determine a uri for
  /// [assetId]. Note that [assetId] may represent a non-importable file such as
  /// a part.
  String _assetIdToUri(AssetId assetId, AssetId from, Element messageTarget) {
    if (!assetId.path.startsWith('lib/')) {
      // Cannot do absolute imports of non lib-based assets.
      if (assetId.package != from.package) {
        _severe(_formatDiagnosticMessage(
            "Attempt to generate non-lib import from different package",
            messageTarget));
        return null;
  
negative.code:   /// update accordingly. The rest of the builder can then rely on every
    /// reflector class to be well-formed, and just have assertions rather than
    /// emitting error messages about it.
    Future<bool> _isReflectorClass(
        ClassElement potentialReflectorClass, ClassElement reflectableClass) async {
      if (potentialReflectorClass == reflectableClass) return false;
      InterfaceType potentialReflectorType = potentialReflectorClass.type;
      InterfaceType reflectableType = reflectableClass.type;
  