Code refactoring (vercel#35937)

Clean up some code in the renderer and make it more readable.

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `yarn lint`

packages/next/server/node-web-streams-helper.ts

@@ -305,28 +305,14 @@ export async function renderToStream({
   generateStaticHTML: boolean
   flushEffectHandler?: () => Promise<string>
 }): Promise<ReadableStream<Uint8Array>> {
-  const closeTag = '</body></html>'
-  const suffixUnclosed = suffix ? suffix.split(closeTag)[0] : null
-  const renderStream: ReadableStream<Uint8Array> & {
-    allReady?: Promise<void>
-  } = await (ReactDOMServer as any).renderToReadableStream(element)
-
-  if (generateStaticHTML) {
-    await renderStream.allReady
-  }
-
-  const transforms: Array<TransformStream<Uint8Array, Uint8Array>> = [
-    createBufferedTransformStream(),
-    flushEffectHandler ? createFlushEffectStream(flushEffectHandler) : null,
-    suffixUnclosed != null ? createPrefixStream(suffixUnclosed) : null,
-    dataStream ? createInlineDataStream(dataStream) : null,
-    suffixUnclosed != null ? createSuffixStream(closeTag) : null,
-  ].filter(Boolean) as any
-
-  return transforms.reduce(
-    (readable, transform) => pipeThrough(readable, transform),
-    renderStream
-  )
+  const renderStream = await renderToInitialStream({ ReactDOMServer, element })
+  return continueFromInitialStream({
+    suffix,
+    dataStream,
+    generateStaticHTML,
+    flushEffectHandler,
+    renderStream,
+  })
 }
 
 export function createSuffixStream(

packages/next/server/render.tsx

@@ -1424,19 +1424,15 @@ export async function renderToHTML(
         }
       }
     } else {
-      let bodyResult
-
-      let renderStream: any
-
       // We start rendering the shell earlier, before returning the head tags
       // to `documentResult`.
       const content = renderContent()
-      renderStream = await renderToInitialStream({
+      const renderStream = await renderToInitialStream({
         ReactDOMServer,
         element: content,
       })
 
-      bodyResult = async (suffix: string) => {
+      const bodyResult = async (suffix: string) => {
         // this must be called inside bodyResult so appWrappers is
         // up to date when getWrappedApp is called