anchor.diff:     * [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
  +   * 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).
      */
  
positive.code:         SourceRange withRange = classElement.withClauseRange;
          errors.add(new AnalysisError.con2(classElement.source, withRange.offset,
              withRange.length, CompileTimeErrorCode.MIXIN_HAS_NO_CONSTRUCTORS,
              [superElement.name]));
          classElement.mixinErrorsReported = true;
        }
        outputs[CONSTRUCTORS] = classElement.constructors;
  
negative.code:     ClassElementImpl classElement = this.target;
      List<ConstructorElement> superConstructors = inputs[SUPER_CONSTRUCTORS];
      DartType superType = classElement.supertype;
      //
      // Shortcut for ClassElement(s) without implicit constructors.
      //
  