MutationObserver + tests

Author: mofeiZ

packages/react-dom/server.node.js

@@ -42,3 +42,7 @@ export function renderToPipeableStream() {
     arguments,
   );
 }
+
+export function getReactDOMClientMutationObserver() {
+  return require('./src/server/ReactDOMClientInstructionObserver').default;
+}