code: function setupProxyProperty(name, opt_setPropsValue) {
      Object.defineProperty(ProxyXMLHttpRequest.prototype, name, {
        'configurable': true,
        'enumerable': true,
        'get': function() {
          return this.handle_[name];
        },
        'set': opt_setPropsValue ? function(value) {
          this.props_[name] = value;
          this.handle_[name] = value;
        } : function(value) {
          this.handle_[name] = value;
        }
      });
    }
docs: Sets up a proxy property, optionally setting a props bag value.
  @param {string} name Property name.
  @param {boolean=} opt_setPropsValue True to set a props value of the same
  name with the given value.