Fill out some more documentation about building RenderBox subclasses.

sky/sdk/lib/rendering/box.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 'dart:sky' as sky;
 
 import 'package:vector_math/vector_math.dart';
 
 import '../base/debug.dart';
@@ skipping 1115 matching lines @@
       paint(canvas);
       sky.view.picture = recorder.endRecording();
     } finally {
       RenderObject.debugDoingPaint = false;
       sky.tracing.end('RenderView.paintFrame');
     }
   }
 
 }
 
-// DEFAULT BEHAVIORS FOR RENDERBOX CONTAINERS
+// HELPER METHODS FOR RENDERBOX CONTAINERS
 abstract class RenderBoxContainerDefaultsMixin<ChildType extends RenderBox, ParentDataType extends ContainerParentDataMixin<ChildType>> implements ContainerRenderObjectMixin<ChildType, ParentDataType> {
 
+  // This class, by convention, doesn't override any members of the superclass.
+  // It only provides helper functions that subclasses can call.
+
   double defaultGetDistanceToFirstActualBaseline(TextBaseline baseline) {
     assert(!needsLayout);
     RenderBox child = firstChild;
     while (child != null) {
       assert(child.parentData is ParentDataType);
       double result = child.getDistanceToActualBaseline(baseline);
       if (result != null)
         return result + child.parentData.position.y;
       child = child.parentData.nextSibling;
     }
@@ skipping 36 matching lines @@
 
   void defaultPaint(RenderCanvas canvas) {
     RenderBox child = firstChild;
     while (child != null) {
       assert(child.parentData is ParentDataType);
       canvas.paintChild(child, child.parentData.position);
       child = child.parentData.nextSibling;
     }
   }
 }