Recover from SSR error in Flight fixture (#28368)

If an error happens before the shell, we need to handle it. In this case
we choose the strategy of rendering a blank document and client
rendering the app. Which will intentionally have a hydration mismatch.

fixtures/flight/server/global.js

@@ -174,8 +174,19 @@ app.all('/', async function (req, res, next) {
       const {pipe} = renderToPipeableStream(React.createElement(Root), {
         bootstrapScripts: mainJSChunks,
         formState: formState,
+        onShellReady() {
+          pipe(res);
+        },
+        onShellError(error) {
+          const {pipe: pipeError} = renderToPipeableStream(
+            React.createElement('html', null, React.createElement('body')),
+            {
+              bootstrapScripts: mainJSChunks,
+            }
+          );
+          pipeError(res);
+        },
       });
-      pipe(res);
     } catch (e) {
       console.error(`Failed to SSR: ${e.stack}`);
       res.statusCode = 500;

fixtures/flight/server/region.js

@@ -103,13 +103,8 @@ app.get('/', async function (req, res) {
 });
 
 app.post('/', bodyParser.text(), async function (req, res) {
-  const {
-    renderToPipeableStream,
-    decodeReply,
-    decodeReplyFromBusboy,
-    decodeAction,
-    decodeFormState,
-  } = await import('react-server-dom-webpack/server');
+  const {decodeReply, decodeReplyFromBusboy, decodeAction, decodeFormState} =
+    await import('react-server-dom-webpack/server');
   const serverReference = req.get('rsc-action');
   if (serverReference) {
     // This is the client-side case