constructor (value) {
    this.value = value;
    this.dep = new Dep();
    this.vmCount = 0;
    def(value, '__ob__', this);
    if (Array.isArray(value)) {
      const augment = hasProto
        ? protoAugment
        : copyAugment;
      augment(value, arrayMethods, arrayKeys);
      this.observeArray(value);
    } else {
      this.walk(value);
    }
  }
Observer class that are attached to each observed
object. Once attached, the observer converts target
object's property keys into getter/setters that
collect dependencies and dispatches updates.