Fill out some more documentation about building RenderBox subclasses.

sky/sdk/lib/base/node.dart

Index: sky/sdk/lib/base/node.dart
diff --git a/sky/sdk/lib/base/node.dart b/sky/sdk/lib/base/node.dart
index e338859349707c3a6c0f8591aab0a1573baf86cf..43acf8adec92aa1f5f823e97dc8c0e688dd8e2d3 100644
--- a/sky/sdk/lib/base/node.dart
+++ b/sky/sdk/lib/base/node.dart
@@ -5,33 +5,7 @@
 class AbstractNode {
 
   // AbstractNode represents a node in a tree.
-  // The AbstractNode protocol is as follows:
-  // - When a subclass is changing the parent of a child, it should
-  //   call either parent.adoptChild(child) or parent.dropChild(child)
-  //   as appropriate. Subclasses should expose an API for
-  //   manipulating the tree if you want to (e.g. a setter for a
-  //   'child' property, or an 'add()' method to manipulate a list).
-  // - You can see the current parent by querying 'parent'.
-  // - You can see the current attachment state by querying
-  //   'attached'. The root of any tree that is to be considered
-  //   attached should be manually attached by calling 'attach()'.
-  //   Other than that, don't call 'attach()' or 'detach()'. This is
-  //   all managed automatically assuming you call the 'adoptChild()'
-  //   and 'dropChild()' methods appropriately.
-  // - Subclasses that have children must override 'attach()' and
-  //   'detach()' as described below.
-  // - Nodes always have a 'depth' greater than their ancestors'.
-  //   There's no guarantee regarding depth between siblings. The
-  //   depth of a node is used to ensure that nodes are processed in
-  //   depth order. The 'depth' of a child can be more than one
-  //   greater than the 'depth' of the parent, because the 'depth'
-  //   values are never decreased: all that matters is that it's
-  //   greater than the parent. Consider a tree with a root node A, a
-  //   child B, and a grandchild C. Initially, A will have 'depth' 0,
-  //   B 'depth' 1, and C 'depth' 2. If C is moved to be a child of A,
-  //   sibling of B, then the numbers won't change. C's 'depth' will
-  //   still be 2. This is all managed automatically assuming you call
-  //   'adoptChild()' and 'dropChild()' appropriately.
+  // The AbstractNode protocol is described in README.md.
 
   int _depth = 0;
   int get depth => _depth;