createMarker_(positionUpdate) {
    if (!this.getMarkerManager_()) {
      throw Error('Cannot create a Marker without Blockly MarkerManager.');
    }
    const position = positionUpdate.position;
    const marker = position.toMarker(this.getWorkspace_());
    marker.colour = this.getColour_();
    this.getMarkerManager_().registerMarker(positionUpdate.workspaceId, marker);
    marker.setCurNode(position.createNode(this.getWorkspace_()));
    return marker;
  }
Create a Marker with a unique color and register it.
@param {!PositionUpdate} positionUpdate The PositionUpdate for which to create
a Marker.
@returns {!Marker} marker The Marker represented by the PositionUpdate.
@throws Throws an error if there is no Blockly MarkerManager to add the
created marker to.
@private