Rename RenderCanvas to PaintingCanvas to avoid confusion with other classes that inherit from Rende…

sky/sdk/example/rendering/touch_demo.dart

Index: sky/sdk/example/rendering/touch_demo.dart
diff --git a/sky/sdk/example/rendering/touch_demo.dart b/sky/sdk/example/rendering/touch_demo.dart
index bbb8a11ba181d46053c997c6ca94c0efb4eacf6a..edd6f129f07306e764fe0eb3069e032230496624 100644
--- a/sky/sdk/example/rendering/touch_demo.dart
+++ b/sky/sdk/example/rendering/touch_demo.dart
@@ -33,7 +33,7 @@ class Dot {
     radius = 5 + (95 * event.pressure);
   }
 
-  void paint(RenderCanvas canvas, Offset offset) {
+  void paint(PaintingCanvas canvas, Offset offset) {
     canvas.drawCircle(position + offset, radius, _paint);
   }
 }
@@ -68,7 +68,7 @@ class RenderTouchDemo extends RenderBox {
     size = constraints.biggest;
   }
 
-  void paint(RenderCanvas canvas, Offset offset) {
+  void paint(PaintingCanvas canvas, Offset offset) {
     Paint white = new Paint()
         ..color = const Color(0xFFFFFFFF);
     canvas.drawRect(offset & size, white);