Avoid white space when resizing map on web (#474)

* Fixes null reference in fullmap example

* Avoid white space when resizing map on web

When resizing the browser screen Flutter fires several resize events.
These events are properly propagated to the containing div, but not
always by the map. It appears Mapbox misses the last resize event.
Therefore, we fire a timer that will check for mismatching width/height
between mapbox and it's container and trigger an additonal resizing.

example/lib/full_map.dart

@@ -32,11 +32,12 @@ class FullMapState extends State<FullMap> {
   Widget build(BuildContext context) {
     return new Scaffold(
         body: MapboxMap(
-          accessToken: MapsDemo.ACCESS_TOKEN,
-          onMapCreated: _onMapCreated,
-          initialCameraPosition:
-          const CameraPosition(target: LatLng(0.0, 0.0)),
-        )
-    );
+      accessToken: MapsDemo.ACCESS_TOKEN,
+      onMapCreated: _onMapCreated,
+      initialCameraPosition: const CameraPosition(target: LatLng(0.0, 0.0)),
+      onStyleLoadedCallback: onStyleLoadedCallback,
+    ));
   }
+
+  void onStyleLoadedCallback() {}
 }

mapbox_gl_web/lib/src/mapbox_map_controller.dart

@@ -341,6 +341,19 @@ class MapboxMapController extends MapboxGlPlatform
     _map.on('movestart', _onCameraMoveStarted);
     _map.on('move', _onCameraMove);
     _map.on('moveend', _onCameraIdle);
+    _map.on('resize', _onMapResize);
+  }
+
+  void _onMapResize(Event e) {
+    Timer(Duration(microseconds: 10), () {
+      var container = _map.getContainer();
+      var canvas = _map.getCanvas();
+      var widthMismatch = canvas.clientWidth != container.clientWidth;
+      var heightMismatch = canvas.clientHeight != container.clientHeight;
+      if (widthMismatch || heightMismatch) {
+        _map.resize();
+      }
+    });
   }
 
   void _onMapClick(e) {