static fromJson(options) {
    // `this` might be a subclass of FieldTemplate if that class doesn't
    // override the static fromJson method.
    return new this(options['value']);
  }
Constructs a FieldTemplate from a JSON arg object.
@param {!Object} options A JSON object with options.
@returns {!FieldTemplate} The new field instance.
@package
@nocollapse