Fill out some more documentation about building RenderBox subclasses.

sky/sdk/lib/rendering/stack.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 'box.dart';
 import 'object.dart';
 
 class StackParentData extends BoxParentData with ContainerParentDataMixin<RenderBox> {
@@ skipping 17 matching lines @@
   bool get isPositioned => top != null || right != null || bottom != null || left != null;
 
   String toString() => '${super.toString()}; top=$top; right=$right; bottom=$bottom, left=$left';
 }
 
 class RenderStack extends RenderBox with ContainerRenderObjectMixin<RenderBox, StackParentData>,
                                          RenderBoxContainerDefaultsMixin<RenderBox, StackParentData> {
   RenderStack({
     List<RenderBox> children
   }) {
-    if (children != null)
-      children.forEach((child) { add(child); });
+    addAll(children);
   }
 
   void setParentData(RenderBox child) {
     if (child.parentData is! StackParentData)
       child.parentData = new StackParentData();
   }
 
 
   double getMinIntrinsicWidth(BoxConstraints constraints) {
     double width = constraints.minWidth;
@@ skipping 144 matching lines @@
   }
 
   void hitTestChildren(HitTestResult result, { Point position }) {
     defaultHitTestChildren(result, position: position);
   }
 
   void paint(RenderCanvas canvas) {
     defaultPaint(canvas);
   }
 }