Fix bugs found by Dart analyzer

sky/sdk/example/rendering/sector_layout.dart

Index: sky/sdk/example/rendering/sector_layout.dart
diff --git a/sky/sdk/example/rendering/sector_layout.dart b/sky/sdk/example/rendering/sector_layout.dart
index 24f09b3b46f308ca76748beb8a50287a6dcc1fa1..52dd265b336c9a81c6d03e806e6114449cd600f5 100644
--- a/sky/sdk/example/rendering/sector_layout.dart
+++ b/sky/sdk/example/rendering/sector_layout.dart
@@ -98,6 +98,8 @@ abstract class RenderSector extends RenderObject {
     assert(sizedByParent);
   }
 
+  Rect get paintBounds => new Rect.fromLTWH(0.0, 0.0, 2.0 * deltaRadius, 2.0 * deltaRadius);

[Hixie, 2015/07/10 18:01:01]

this seems wrong. the origin is on the middle of the circle. this should
probably be from -radius-deltaRadius to radius+deltaRadius.

[abarth-chromium, 2015/07/10 18:09:30]

Ok.  Currently it's not used by anything because none of the sectors create
their own display list.

+
   bool hitTest(HitTestResult result, { double radius, double theta }) {
     assert(parentData is SectorParentData);
     if (radius < parentData.radius || radius >= parentData.radius + deltaRadius ||
@@ -492,7 +494,7 @@ class RenderBoxToRenderSectorAdapter extends RenderBox {
     result.add(new BoxHitTestEntry(this, position));
     return true;
   }
-  
+
 }
 
 class RenderSolidColor extends RenderDecoratedSector {