static fromFieldChangeEvent_(event) {
    const type = 'FIELD';
    const blockId = event.blockId;
    const fieldName = event.name;
    return new Position(type, blockId, fieldName);
  }
Create a Position from a Blockly Change event.
@param {!Blockly.Event.Change} event The event that creates a Position.
@returns {!PositionUpdate} The Position representative of the event.
@private