anchor.diff:        InterfaceType superType, void callback(
             ConstructorElement explicitConstructor, List<DartType> parameterTypes,
             List<DartType> argumentTypes)) {
  +    if (superType == null) {
  +      return false;
  +    }
       ClassElement superclassElement = superType.element;
       List<ConstructorElement> constructors = superclassElement.constructors;
       int count = constructors.length;
  
positive.code:     ClassElementImpl classElement = this.target;
      List<ConstructorElement> superConstructors = inputs[SUPER_CONSTRUCTORS];
      DartType superType = classElement.supertype;
      ClassElement superElement = superType.element;
      //
      // Shortcut for ClassElement(s) without implicit constructors.
      //
  
negative.code:         SourceRange withRange = classElement.withClauseRange;
          errors.add(new AnalysisError.con2(classElement.source, withRange.offset,
              withRange.length, CompileTimeErrorCode.MIXIN_HAS_NO_CONSTRUCTORS,
              [superType.element.name]));
          classElement.mixinErrorsReported = true;
        }
        outputs[CONSTRUCTORS] = classElement.constructors;
  