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:    * [superType], to the class or mixin application [classElement],
     * and pass information about them to [callback].
     *
     * Return true if some constructors were considered.  (A false return value
     * can only happen if the supeclass is a built-in type, in which case it
     * can't be used as a mixin anyway).
     */
  
negative.code:           errors.add(new AnalysisError.con2(classElement.source,
                classElement.nameOffset, classElement.name.length,
                CompileTimeErrorCode.MIXIN_HAS_NO_CONSTRUCTORS,
                [superType.element.name]));
          } else {
            classElement.constructors = implicitConstructors;
          }
  