Make checkbox widget to fit more closely to material design standard. Make...

Make checkbox widget to fit more closely to  material design standard. Make changes to remove animation artefacts.

packages/flutter/lib/src/widgets/checkbox.dart

@@ -15,8 +15,9 @@ export 'package:sky/src/rendering/toggleable.dart' show ValueChanged;
 const double _kMidpoint = 0.5;
 const sky.Color _kLightUncheckedColor = const sky.Color(0x8A000000);
 const sky.Color _kDarkUncheckedColor = const sky.Color(0xB2FFFFFF);
-const double _kEdgeSize = 20.0;
+const double _kEdgeSize = 18.0;
 const double _kEdgeRadius = 1.0;
+const double _kStrokeWidth = 2.0;
 
 /// A material design checkbox
 ///
@@ -127,13 +128,18 @@ class _RenderCheckbox extends RenderToggleable {
     final PaintingCanvas canvas = context.canvas;
     // Choose a color between grey and the theme color
     sky.Paint paint = new sky.Paint()
-      ..strokeWidth = 2.0
+      ..strokeWidth = _kStrokeWidth
       ..color = uncheckedColor;
 
-    // The rrect contracts slightly during the animation
+    // The rrect contracts slightly during the transition animation from checked states.
+    // Because we have a stroke size of 2, we should have a minimum 1.0 inset.
     double inset = 2.0 - (position - _kMidpoint).abs() * 2.0;
-    sky.Rect rect = new sky.Rect.fromLTWH(offset.dx + inset, offset.dy + inset,
-        _kEdgeSize - inset, _kEdgeSize - inset);
+    double rectSize = _kEdgeSize - inset * _kStrokeWidth;
+    sky.Rect rect =
+      new sky.Rect.fromLTWH(offset.dx + inset, offset.dy + inset, rectSize, rectSize);
+    // Create an inner rectangle to cover inside of rectangle. This is needed to avoid
+    // painting artefacts caused by overlayed paintings.
+    sky.Rect innerRect = rect.deflate(1.0);
     sky.RRect rrect = new sky.RRect()
       ..setRectXY(rect, _kEdgeRadius, _kEdgeRadius);
 
@@ -150,25 +156,27 @@ class _RenderCheckbox extends RenderToggleable {
         const sky.Color(0x00000000),
         uncheckedColor
       ]));
-      canvas.drawRRect(rrect, paint);
+      canvas.drawRect(innerRect, paint);
     }
 
     if (position > _kMidpoint) {
       double t = (position - _kMidpoint) / (1.0 - _kMidpoint);
 
-      // Solid filled rrect
-      paint.setStyle(sky.PaintingStyle.strokeAndFill);
+      // First draw a rounded rect outline then fill inner rectangle with accent color.
       paint.color = new Color.fromARGB((t * 255).floor(), _accentColor.red,
           _accentColor.green, _accentColor.blue);
+      paint.setStyle(sky.PaintingStyle.stroke);
       canvas.drawRRect(rrect, paint);
+      paint.setStyle(sky.PaintingStyle.fill);
+      canvas.drawRect(innerRect, paint);
 
       // White inner check
       paint.color = const sky.Color(0xFFFFFFFF);
       paint.setStyle(sky.PaintingStyle.stroke);
       sky.Path path = new sky.Path();
-      sky.Point start = new sky.Point(_kEdgeSize * 0.2, _kEdgeSize * 0.5);
+      sky.Point start = new sky.Point(_kEdgeSize * 0.15, _kEdgeSize * 0.45);
       sky.Point mid = new sky.Point(_kEdgeSize * 0.4, _kEdgeSize * 0.7);
-      sky.Point end = new sky.Point(_kEdgeSize * 0.8, _kEdgeSize * 0.3);
+      sky.Point end = new sky.Point(_kEdgeSize * 0.85, _kEdgeSize * 0.25);
       Point lerp(Point p1, Point p2, double t) =>
           new Point(p1.x * (1.0 - t) + p2.x * t, p1.y * (1.0 - t) + p2.y * t);
       sky.Point drawStart = lerp(start, mid, 1.0 - t);