Fix the assertion failure with inserting node contains child which has id into the document

https://github.com/mozilla/servo/issues/2630

src/components/script/dom/node.rs

@@ -1137,10 +1137,13 @@ impl Node {
         // Step 8.
         for node in nodes.mut_iter() {
             parent.add_child(node, child);
-            if parent.is_in_doc() {
+            let is_in_doc = parent.is_in_doc();
-                node.flags.deref().borrow_mut().insert(IsInDoc);
+            for mut kid in node.traverse_preorder() {
+                if is_in_doc {
+                    kid.flags.deref().borrow_mut().insert(IsInDoc);
                 } else {
-                node.flags.deref().borrow_mut().remove(IsInDoc);
+                    kid.flags.deref().borrow_mut().remove(IsInDoc);
+                }
             }
         }
 

src/test/content/test_document_getElementById.html

@@ -106,6 +106,18 @@
       is(target2, null, "test 8-1, should return null after updated id via Attr.value");
     }
 
+    // Test the assertion with inserting node with child having id into the document (mozilla#2630)
+    // This need not to port to WPF-test because this tests servo's internally flags.
+    {
+      let TEST_ID = "test-9";
+      let a = document.createElement("a");
+      let b = document.createElement("b");
+      a.appendChild(b).id = TEST_ID;
+      gBody.appendChild(a);
+      let result = document.getElementById(TEST_ID);
+      is(result, b, "test 9-0");
+    }
+
     finish();
   </script>
 </body>