Add RenderClip and use it in FixedHeightScrollable

In the future, we might want to extend this class to clip to a configurable
rect in addition to clipping to its bounds.

R=eseidel@chromium.org, ianh@google.com

Review URL: https://codereview.chromium.org/1145263006

sdk/lib/framework/components2/fixed_height_scrollable.dart

@@ -70,14 +70,11 @@ abstract class FixedHeightScrollable extends Scrollable {
       }
     }
 
-    // TODO(abarth): Add a clip.
-    return new BlockContainer(
-      children: [
-        new Transform(
-          transform: transform,
-          child: new BlockContainer(children: buildItems(itemNumber, drawCount))
-        )
-      ]
+    return new Clip(
+      child: new Transform(
+        transform: transform,
+        child: new BlockContainer(children: buildItems(itemNumber, drawCount))
+      )
     );
   }
 

sdk/lib/framework/fn2.dart

@@ -352,6 +352,15 @@ abstract class OneChildRenderObjectWrapper extends RenderObjectWrapper {
   }
 }
 
+class Clip extends OneChildRenderObjectWrapper {
+  RenderClip root;
+
+  Clip({ UINode child, Object key })
+    : super(child: child, key: key);
+
+  RenderClip createNode() => new RenderClip();
+}
+
 class Padding extends OneChildRenderObjectWrapper {
   RenderPadding root;
   final EdgeDims padding;

sdk/lib/framework/rendering/box.dart

@@ -215,6 +215,19 @@ class RenderSizedBox extends RenderProxyBox {
   }
 }
 
+class RenderClip extends RenderProxyBox {
+  RenderClip({ RenderBox child }) : super(child);
+
+  void paint(RenderObjectDisplayList canvas) {
+    if (child != null) {
+      canvas.save();
+      canvas.clipRect(new sky.Rect.fromSize(size));
+      child.paint(canvas);
+      canvas.restore();
+    }
+  }
+}
+
 class RenderPadding extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
 
   RenderPadding({ EdgeDims padding, RenderBox child }) {