Implementation of isReactive()

In fact, we have already distinguished between reactive objects and readonly objects in createGetter implementation:

// higher-order function
function createGetter(isReadonly = false) {
  return function get(target, key) {
    const res = Reflect.get(target, key);
    if (!isReadonly) {
      track(target, key);
    }
    return res;
  };
}

So, we just need to trigger get operation.

The same applies to isReadonly().