function enableClassCheck(target) {
  var classAttr = ['__\0is_clz', classBase++].join('_');
  target.prototype[classAttr] = true;

  if (true) {
    assert(!target.isInstance, 'The method "is" can not be defined.');
  }

  target.isInstance = function (obj) {
    return !!(obj && obj[classAttr]);
  };
} // superCall should have class info, which can not be fetch from 'this'.
Implements `CheckableConstructor` for `target`.
Can not use instanceof, consider different scope by
cross domain or es module import in ec extensions.
Mount a method "isInstance()" to Clz.

@usage
```ts
class Xxx {}
type XxxConstructor = typeof Xxx & CheckableConstructor;
enableClassCheck(Xxx as XxxConstructor)
```