_definePlainAttribute(attr) {
    // Ensure the property hasn't already been defined
    let existingProperty = Object.getOwnPropertyDescriptor(this, attr);
    if (existingProperty && existingProperty.get) {
      return;
    }

    // Ensure the attribute is on the attrs hash
    if (!Object.prototype.hasOwnProperty.call(this.attrs, attr)) {
      this.attrs[attr] = null;
    }

    // Define the getter/setter
    Object.defineProperty(this, attr, {
      get() {
        return this.attrs[attr];
      },
      set(val) {
        this.attrs[attr] = val;
      },
    });
  }
Define getter/setter for a plain attribute
    @method _definePlainAttribute
    @param attr
    @private
    @hide