Merge pull request facebook#879 from syranide/inlinechild

Remove unnecessary tests from insertChildAt and inline it instead

src/browser/ui/dom/DOMChildrenOperations.js

@@ -42,20 +42,14 @@ var textContentAccessor = getTextContentAccessor();
  * @internal
  */
 function insertChildAt(parentNode, childNode, index) {
-  var childNodes = parentNode.childNodes;
-  if (childNodes[index] === childNode) {
-    return;
-  }
-  // If `childNode` is already a child of `parentNode`, remove it so that
-  // computing `childNodes[index]` takes into account the removal.
-  if (childNode.parentNode === parentNode) {
-    parentNode.removeChild(childNode);
-  }
-  if (index >= childNodes.length) {
-    parentNode.appendChild(childNode);
-  } else {
-    parentNode.insertBefore(childNode, childNodes[index]);
-  }
+  // By exploiting arrays returning `undefined` for an undefined index, we can
+  // rely exclusively on `insertBefore(node, null)` instead of also using
+  // `appendChild(node)`. However, using `undefined` is not allowed by all
+  // browsers so we must replace it with `null`.
+  parentNode.insertBefore(
+    childNode,
+    parentNode.childNodes[index] || null
+  );
 }
 
 var updateTextContent;