Ensure that sky Nodes are in the document when didMount() is fired. Refactor FixedHeightScrollable …

sky/framework/fn.dart

Index: sky/framework/fn.dart
diff --git a/sky/framework/fn.dart b/sky/framework/fn.dart
index c921a4c2f48fbfb51bada7d6b85d545cf2d76346..380dc52ebf9a37f6ea68ae70cf97700745b7e5dc 100644
--- a/sky/framework/fn.dart
+++ b/sky/framework/fn.dart
@@ -247,6 +247,7 @@ abstract class Element extends Node {
       // print("...no oldElement, initial render");
 
       _root = sky.document.createElement(_tagName);
+      _parentInsertBefore(host, _root, insertBefore);
       _syncNode();
 
       for (var child in _children) {
@@ -254,7 +255,6 @@ abstract class Element extends Node {
         assert(child._root is sky.Node);
       }
 
-      _parentInsertBefore(host, _root, insertBefore);
       return false;
     }