function proxy (target, sourceKey, key) {
  sharedPropertyDefinition.get = function proxyGetter () {
    return this[sourceKey][key]
  };
  sharedPropertyDefinition.set = function proxySetter (val) {
    this[sourceKey][key] = val;
  };
  Object.defineProperty(target, key, sharedPropertyDefinition);
}
Remove self from all dependencies' subscriber list.