[AutoLayout] Address further concerns raised in https://codereview.chromium.org/1230583003

sky/sdk/example/rendering/simple_autolayout.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:sky';
 import 'package:sky/rendering/box.dart';
 import 'package:sky/rendering/object.dart';
 import 'package:sky/rendering/sky_binding.dart';
 import 'package:sky/rendering/auto_layout.dart';
 import 'package:cassowary/cassowary.dart' as al;
 
 void main() {
-  var c1 = new RenderDecoratedBox(
+  RenderDecoratedBox c1 = new RenderDecoratedBox(
     decoration: new BoxDecoration(backgroundColor: const Color(0xFFFF0000))
   );
 
-  var c2 = new RenderDecoratedBox(
+  RenderDecoratedBox c2 = new RenderDecoratedBox(
     decoration: new BoxDecoration(backgroundColor: const Color(0xFF00FF00))
   );
 
-  var c3 = new RenderDecoratedBox(
+  RenderDecoratedBox c3 = new RenderDecoratedBox(
     decoration: new BoxDecoration(backgroundColor: const Color(0xFF0000FF))
   );
 
-  var c4 = new RenderDecoratedBox(
+  RenderDecoratedBox c4 = new RenderDecoratedBox(
     decoration: new BoxDecoration(backgroundColor: const Color(0xFFFFFFFF))
   );
 
-  var root = new RenderAutoLayout(children: [c1, c2, c3, c4]);
+  RenderAutoLayout root = new RenderAutoLayout(children: [c1, c2, c3, c4]);
 
   AutoLayoutParentData p1 = c1.parentData;
   AutoLayoutParentData p2 = c2.parentData;
   AutoLayoutParentData p3 = c3.parentData;
   AutoLayoutParentData p4 = c4.parentData;
 
   root.addConstraints(<al.Constraint>[
     // Sum of widths of each box must be equal to that of the container
     (p1.width + p2.width + p3.width == root.width) as al.Constraint,
 
@@ skipping 16 matching lines @@
     // The fourth box should be half as wide as the second and must be attached
     // to the right edge of the same (by its center)
     (p4.width == p2.width / al.cm(2.0)) as al.Constraint,
     (p4.height == al.cm(50.0)) as al.Constraint,
     (p4.horizontalCenter == p2.rightEdge) as al.Constraint,
     (p4.verticalCenter == p2.height / al.cm(2.0)) as al.Constraint,
   ]);
 
   new SkyBinding(root: root);
 }