Fill out some more documentation about building RenderBox subclasses.

sky/sdk/lib/rendering/block.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 BlockParentData extends BoxParentData with ContainerParentDataMixin<RenderBox> { }
 
 class RenderBlock extends RenderBox with ContainerRenderObjectMixin<RenderBox, BlockParentData>,
                                          RenderBoxContainerDefaultsMixin<RenderBox, BlockParentData> {
   // lays out RenderBox children in a vertical stack
   // uses the maximum width provided by the parent
   // sizes itself to the height of its child stack
 
   RenderBlock({
     List<RenderBox> children
   }) {
-    if (children != null)
-      children.forEach((child) { add(child); });
+    addAll(children);
   }
 
   void setParentData(RenderBox child) {
     if (child.parentData is! BlockParentData)
       child.parentData = new BlockParentData();
   }
 
   double getMinIntrinsicWidth(BoxConstraints constraints) {
     double width = 0.0;
     BoxConstraints innerConstraints = new BoxConstraints(
@@ skipping 70 matching lines @@
   void hitTestChildren(HitTestResult result, { Point position }) {
     defaultHitTestChildren(result, position: position);
   }
 
   void paint(RenderCanvas canvas) {
     defaultPaint(canvas);
   }
 
 }