async function getPositionUpdates(workspaceId) {
  return new Promise((resolve, reject) => {
    socket.emit('getPositionUpdates', workspaceId, (positionUpdates) => {
      positionUpdates.forEach((positionUpdate) => {
        positionUpdate.position = Position.fromJson(positionUpdate.position);
      });
      resolve(positionUpdates);
    });
  });
}
Get the position for a given user. If no user is specified will return the
positions of all users.
@param {string=} workspaceId workspaceId of the user.
@returns {!Promise} Promise object with an array of PositionUpdate objects.
@public