Prevent already scheduled-for-render fn components which become removed from rendering

sky/framework/fn.dart

Index: sky/framework/fn.dart
diff --git a/sky/framework/fn.dart b/sky/framework/fn.dart
index dbf193ab04bfd2e9cf116267bcc7430a0102d864..c921a4c2f48fbfb51bada7d6b85d545cf2d76346 100644
--- a/sky/framework/fn.dart
+++ b/sky/framework/fn.dart
@@ -636,13 +636,17 @@ abstract class Component extends Node {
   }
 
   void _renderIfDirty() {
+    if (_removed)
+      return;
+
     assert(_rendered != null);
-    assert(!_removed);
 
     var rendered = _rendered;
     while (rendered is Component) {
       rendered = rendered._rendered;
     }
+
+    assert(rendered._root != null);
     sky.Node root = rendered._root;
 
     _renderInternal(root.parentNode, root.nextSibling);