Add RenderClip and use it in FixedHeightScrollable

sky/sdk/lib/framework/rendering/box.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 import 'dart:typed_data';
 import 'object.dart';
 import 'package:vector_math/vector_math.dart';
 import 'package:sky/framework/net/image_cache.dart' as image_cache;
@@ skipping 199 matching lines @@
     return constraints.constrain(_desiredSize);
   }
 
   void performLayout() {
     size = constraints.constrain(_desiredSize);
     if (child != null)
       child.layout(new BoxConstraints.tight(size));
   }
 }
 
+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 }) {
     assert(padding != null);
     this.padding = padding;
     this.child = child;
   }
 
   EdgeDims _padding;
   EdgeDims get padding => _padding;
@@ skipping 426 matching lines @@
 
   void defaultPaint(RenderObjectDisplayList canvas) {
     RenderBox child = firstChild;
     while (child != null) {
       assert(child.parentData is ParentDataType);
       canvas.paintChild(child, child.parentData.position);
       child = child.parentData.nextSibling;
     }
   }
 }