The container class for MapControls. There's exactly one ControlsPane in a MapViewport.

class ControlsPane {
 MapViewport _viewport;
 DivElement _root;

 final List<MapControl> controls = [];

 ControlsPane() {
   _root = new Element.tag("div");
 }

 /// the root DOM element for this control
 DivElement get root => _root;

 /// Detach this pane from the map viewport.
 void detach() {
   _viewport = null;
 }

 /// Attach this control pane to the [viewport]. [viewport]
 /// must not be null.
 void attach(MapViewport viewport) {
   _require(viewport != null, "viewport must not be null");
   _viewport = viewport;
   _root.style
     ..width = "100%"
     ..height = "100%"
     ..position = "absolute"
     ..top = "0px"
     ..left = "0px";
   // Don't add the _container to the viewport container,
   // the map viewport takes care of this
 }

 /// layout the contronls pane
 void layout() {
   controls.forEach((c) => c.layout());
 }
}