[Flight] Move Flight DOM to a Webpack Specific Package (#17372)

* Move Flight DOM to Webpack Specific Packagee

We'll have Webpack specific coupling so we need to ensure that it can be
versioned separately from various Webpack versions. We'll also have builds
for other bundlers in the future.

* Move to peerDep

* Move DOM Flight Tests

* Merge ReactFlightIntegration into ReactFlightDOM

This was an integration test. We can add to it.

* Fix fixture paths

packages/react-server/src/ReactDOMServerFormatConfig.js

@@ -0,0 +1,35 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+import {convertStringToBuffer} from 'react-server/src/ReactServerHostConfig';
+
+import ReactDOMServer from 'react-dom/server';
+
+export function formatChunkAsString(type: string, props: Object): string {
+  let str = '<' + type + '>';
+  if (typeof props.children === 'string') {
+    str += props.children;
+  }
+  str += '</' + type + '>';
+  return str;
+}
+
+export function formatChunk(type: string, props: Object): Uint8Array {
+  return convertStringToBuffer(formatChunkAsString(type, props));
+}
+
+export function renderHostChildrenToString(
+  children: React$Element<any>,
+): string {
+  // TODO: This file is used to actually implement a server renderer
+  // so we can't actually reference the renderer here. Instead, we
+  // should replace this method with a reference to Fizz which
+  // then uses this file to implement the server renderer.
+  return ReactDOMServer.renderToStaticMarkup(children);
+}