code: function(terria, scene, handler, keyboardModifier) {
    this._terria = terria;
    this._scene = scene;
    this._ellipsoid = scene.globe.ellipsoid;
    this._finishHandler = handler;
    this._mouseHandler = new ScreenSpaceEventHandler(scene.canvas, false);
    this._stopHandler = undefined;
    this._interHandler = undefined;
    this._dataSource = undefined;
    this._click1 = undefined;
    this._click2 = undefined;
    this.active = false;
    this.keyboardModifier = defaultValue(
      keyboardModifier,
      KeyboardEventModifier.SHIFT
    );
  }
docs: ------------------------------------------- DrawExtentHelper from the cesium sample code modified to be available with a keyboard modifier on left-click - default is SHIFT -------------------------------------------