getMarker(workspaceId) {
    return this.getMarkerManager_()
      ? this.getMarkerManager_().getMarker(workspaceId)
      : null;
  }
Get the Marker that corresponds to the given user.
@param {string} workspaceId The workspaceId of the user.
@returns {Blockly.Marker} The Marker for the user if it exists, otherwise
null.
@private