Make RRect immutable and replace RRect.setRectXY with a fromRectXY constructor

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

@@ -151,8 +151,8 @@ class _RenderCheckbox extends RenderToggleable {
     // Create an inner rectangle to cover inside of rectangle. This is needed to avoid
     // painting artefacts caused by overlayed paintings.
     Rect innerRect = rect.deflate(1.0);
-    ui.RRect rrect = new ui.RRect()
-      ..setRectXY(rect, _kEdgeRadius, _kEdgeRadius);
+    ui.RRect rrect = new ui.RRect.fromRectXY(
+        rect, _kEdgeRadius, _kEdgeRadius);
 
     // Outline of the empty rrect
     paint.style = ui.PaintingStyle.stroke;

packages/flutter/lib/src/material/switch.dart

@@ -124,8 +124,8 @@ class _RenderSwitch extends RenderToggleable {
     Rect rect = new Rect.fromLTWH(offset.dx,
         offset.dy + _kSwitchHeight / 2.0 - _kTrackHeight / 2.0, _kTrackWidth,
         _kTrackHeight);
-    ui.RRect rrect = new ui.RRect()
-      ..setRectXY(rect, _kTrackRadius, _kTrackRadius);
+    ui.RRect rrect = new ui.RRect.fromRectXY(
+        rect, _kTrackRadius, _kTrackRadius);
     canvas.drawRRect(rrect, paint);
 
     if (_radialReaction != null)

packages/flutter/lib/src/painting/box_painter.dart

@@ -949,7 +949,7 @@ class BoxPainter {
             canvas.drawRect(rect, _backgroundPaint);
           } else {
             double radius = _getEffectiveBorderRadius(rect);
-            canvas.drawRRect(new ui.RRect()..setRectXY(rect, radius, radius), _backgroundPaint);
+            canvas.drawRRect(new ui.RRect.fromRectXY(rect, radius, radius), _backgroundPaint);
           }
           break;
       }
@@ -1043,8 +1043,8 @@ class BoxPainter {
     double width = _decoration.border.top.width;
     double radius = _getEffectiveBorderRadius(rect);
 
-    ui.RRect outer = new ui.RRect()..setRectXY(rect, radius, radius);
-    ui.RRect inner = new ui.RRect()..setRectXY(rect.deflate(width), radius - width, radius - width);
+    ui.RRect outer = new ui.RRect.fromRectXY(rect, radius, radius);
+    ui.RRect inner = new ui.RRect.fromRectXY(rect.deflate(width), radius - width, radius - width);
     canvas.drawDRRect(outer, inner, new Paint()..color = color);
   }
 

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -587,7 +587,7 @@ class RenderClipRRect extends RenderProxyBox {
   void paint(PaintingContext context, Offset offset) {
     if (child != null) {
       Rect rect = offset & size;
-      ui.RRect rrect = new ui.RRect()..setRectXY(rect, xRadius, yRadius);
+      ui.RRect rrect = new ui.RRect.fromRectXY(rect, xRadius, yRadius);
       context.paintChildWithClipRRect(child, offset.toPoint(), rect, rrect);
     }
   }