function mountExtend(SubClz, SupperClz) {
  SubClz.extend = SupperClz.extend;
} // A random offset.
A work around to both support ts extend and this extend mechanism.
on sub-class.
@usage
```ts
class Component { ... }
classUtil.enableClassExtend(Component);
classUtil.enableClassManagement(Component, {registerWhenExtend: true});

class Series extends Component { ... }
// Without calling `markExtend`, `registerWhenExtend` will not work.
Component.markExtend(Series);
```