static fromSelectedUiEvent_(event) {
    // Assume this is a block selected event because workspace comments (also
    // selectable) are disabled.
    const type = 'BLOCK';
    const blockId = event.newElementId;
    const fieldName = null;
    return new Position(type, blockId, fieldName);
  }
Create a Position from a Blockly UI event.
@param {!Blockly.Events.Selected} event The event that creates a Position.
@returns {!PositionUpdate} The Position representative of the event.
@private