tick(time, timeDelta) {
    var i;
    var systems = this.systems;

    // Components.
    this.callComponentBehaviors('tick', time, timeDelta);

    // Systems.
    for (i = 0; i < this.systemNames.length; i++) {
      if (!systems[this.systemNames[i]].tick) {
        continue;
      }
      systems[this.systemNames[i]].tick(time, timeDelta);
    }
  }
Behavior-updater meant to be called from scene render.
Abstracted to a different function to facilitate unit testing (`scene.tick()`) without
needing to render.