Fix logic in RenderPadding.

sky/tests/raw/box_layout.dart

+import '../resources/third_party/unittest/unittest.dart';
+import '../resources/unit.dart';
+import '../resources/display_list.dart';
+import 'dart:math' as math;
+import 'dart:sky' as sky;
+import 'package:sky/framework/app.dart';
+import 'package:sky/framework/rendering/box.dart';
+import 'package:sky/framework/rendering/block.dart';
+import 'package:sky/framework/rendering/node.dart';
+
+TestApp app;
+
+void main() {
+  initUnit();
+
+  test("padding", () {
+    var size = new RenderSizedBox(desiredSize: new sky.Size(double.INFINITY, 100.0));
+    var inner = new RenderDecoratedBox(decoration: new BoxDecoration(backgroundColor: 0xFF00FF00), child: size);
+    var padding = new RenderPadding(padding: new EdgeDims.all(50.0), child: inner);
+    var block = new RenderBlock(children: [padding]);
+    var outer = new RenderDecoratedBox(decoration: new BoxDecoration(backgroundColor: 0xFF0000FF), child: block);
+    app = new TestApp(outer);
+  });
+}