[Flight] Simulate fetch to third party in fixture (#33484)

This adds some I/O to go get the third party thing to test how it
overlaps.

With #33482, this is what it looks like. The await gets cut off when the
third party component starts rendering. I.e. after the latency to start.

<img width="735" alt="Screenshot 2025-06-08 at 5 42 46 PM"
src="https://github.com/user-attachments/assets/f68d9a84-05a1-4125-b3f0-8f3e4eaaa5c1"
/>

This doesn't fully simulate everything because it should actually also
simulate each chunk of the stream coming back too. We could wrap the
ReadableStream to simulate that. In that scenario, it would probably get
some awaits on the chunks at the end too.

Author: sebmarkbage

fixtures/flight/src/App.js

@@ -43,7 +43,7 @@ async function Bar({children}) {
 }
 
 async function ThirdPartyComponent() {
-  return delay('hello from a 3rd party', 30);
+  return await delay('hello from a 3rd party', 30);
 }
 
 let cachedThirdPartyStream;
@@ -52,16 +52,35 @@ let cachedThirdPartyStream;
 // That way it gets the owner from the call to createFromNodeStream.
 const thirdPartyComponent = <ThirdPartyComponent />;
 
-function fetchThirdParty(noCache) {
+function simulateFetch(cb, latencyMs) {
+  return new Promise(resolve => {
+    // Request latency
+    setTimeout(() => {
+      const result = cb();
+      // Response latency
+      setTimeout(() => {
+        resolve(result);
+      }, latencyMs);
+    }, latencyMs);
+  });
+}
+
+async function fetchThirdParty(noCache) {
   // We're using the Web Streams APIs for tee'ing convenience.
-  const stream =
-    cachedThirdPartyStream && !noCache
-      ? cachedThirdPartyStream
-      : renderToReadableStream(
+  let stream;
+  if (cachedThirdPartyStream && !noCache) {
+    stream = cachedThirdPartyStream;
+  } else {
+    stream = await simulateFetch(
+      () =>
+        renderToReadableStream(
           thirdPartyComponent,
           {},
           {environmentName: 'third-party'}
-        );
+        ),
+      25
+    );
+  }
 
   const [stream1, stream2] = stream.tee();
   cachedThirdPartyStream = stream1;