Small fixes and nits to make null-safe simpler (#686)

doc/examples/gestures/lib/main_multitap.dart

@@ -15,14 +15,8 @@ void main() {
 class MyGame extends BaseGame with MultiTouchTapDetector {
   final _whitePaint = BasicPalette.white.paint;
 
-  Paint _paint;
-
   final Map<int, Rect> _taps = {};
 
-  MyGame() {
-    _paint = _whitePaint;
-  }
-
   @override
   void onTapDown(int pointerId, TapDownDetails details) {
     _taps[pointerId] = Rect.fromLTWH(
@@ -47,7 +41,7 @@ class MyGame extends BaseGame with MultiTouchTapDetector {
   void render(Canvas canvas) {
     super.render(canvas);
     _taps.values.forEach((rect) {
-      canvas.drawRect(rect, _paint);
+      canvas.drawRect(rect, _whitePaint);
     });
   }
 }