Rationalize Dart mojo and sky package structure

sky/sdk/lib/framework/node.dart

 library node;
 
 class Node {
 
   // 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.
@@ skipping 40 matching lines @@
     redepthChild(child);
   }
   void dropChild(Node child) { // only for use by subclasses
     assert(child != null);
     assert(child.attached == attached);
     if (attached)
       child.detach();
   }
 
 }