Sector demo is broken because of compositor changes

We need to implement visitChildren for the RenderObjects in the sector demo.
Also, add a test.

Fixes #790

examples/rendering/sector_layout.dart

@@ -166,6 +166,14 @@ class RenderSectorWithChildren extends RenderDecoratedSector with ContainerRende
       child = child.parentData.previousSibling;
     }
   }
+
+  void visitChildren(RenderObjectVisitor visitor) {
+    RenderSector child = lastChild;
+    while (child != null) {
+      visitor(child);
+      child = child.parentData.previousSibling;
+    }
+  }
 }
 
 class RenderSectorRing extends RenderSectorWithChildren {
@@ -413,6 +421,10 @@ class RenderBoxToRenderSectorAdapter extends RenderBox {
       child.parentData = new SectorParentData();
   }
 
+  void visitChildren(RenderObjectVisitor visitor) {
+    visitor(_child);
+  }
+
   double getMinIntrinsicWidth(BoxConstraints constraints) {
     if (child == null)
       return super.getMinIntrinsicWidth(constraints);

packages/unit/test/examples/sector_layout_test.dart

+import 'package:test/test.dart';
+import '../rendering/layout_utils.dart';
+import '../../../../examples/rendering/sector_layout.dart';
+
+void main() {
+  test('Sector layout can paint', () {
+    RenderingTester tester = new RenderingTester(root: buildSectorExample());
+    tester.pumpFrame();
+  });
+}

packages/unit/test/rendering/layout_utils.dart

@@ -7,14 +7,36 @@ class TestRenderView extends RenderView {
     attach();
     rootConstraints = new ViewConstraints(size: _kTestViewSize);
     scheduleInitialLayout();
+    scheduleInitialPaint(new TransformLayer(transform: new Matrix4.identity()));
   }
+}
+
+enum EnginePhase {
+  layout,
+  paint,
+  composite
+}
+
+class RenderingTester {
+  RenderingTester({ RenderBox root }) {
+    renderView = new TestRenderView(child: root);
+  }
+
+  RenderView renderView;
 
-  void beginFrame(double timeStamp) {
+  void pumpFrame({ EnginePhase phase: EnginePhase.composite }) {
     RenderObject.flushLayout();
+    if (phase == EnginePhase.layout)
+      return;
+    renderView.updateCompositingBits();
+    RenderObject.flushPaint();
+    if (phase == EnginePhase.paint)
+      return;
+    renderView.compositeFrame();
   }
 }
 
-RenderView layout(RenderBox box, { BoxConstraints constraints }) {
+RenderingTester layout(RenderBox box, { BoxConstraints constraints }) {
   if (constraints != null) {
     box = new RenderPositionedBox(
       child: new RenderConstrainedBox(
@@ -24,7 +46,7 @@ RenderView layout(RenderBox box, { BoxConstraints constraints }) {
     );
   }
 
-  TestRenderView renderView = new TestRenderView(child: box);
-  renderView.beginFrame(0.0);
-  return renderView;
+  RenderingTester tester = new RenderingTester(root: box);
+  tester.pumpFrame(phase: EnginePhase.layout);
+  return tester;
 }

packages/unit/test/rendering/viewport_test.dart

@@ -15,16 +15,16 @@ void main() {
       child: size);
 
     RenderViewport viewport = new RenderViewport(child: red, scrollOffset: new Offset(0.0, -10.0));
-    RenderView renderView = layout(viewport);
+    RenderingTester tester = layout(viewport);
 
     HitTestResult result;
 
     result = new HitTestResult();
-    renderView.hitTest(result, position: new Point(15.0, 0.0));
+    tester.renderView.hitTest(result, position: new Point(15.0, 0.0));
     expect(result.path.first.target, equals(viewport));
 
     result = new HitTestResult();
-    renderView.hitTest(result, position: new Point(15.0, 15.0));
+    tester.renderView.hitTest(result, position: new Point(15.0, 15.0));
     expect(result.path.first.target, equals(size));
   });
 }