[Flight] Don't use object property initializer for async iterable (#33591)

It turns out this was being compiled to a `_defineProperty` helper by
Babel or Closure. We're supposed to have it error the build when we use
features like this that might get compiled.

We should stick to simple ES5 features.

Author: sebmarkbage

packages/react-client/src/ReactFlightClient.js

@@ -2077,32 +2077,34 @@ function startAsyncIterable<T>(
       }
     },
   };
-  const iterable: $AsyncIterable<T, T, void> = {
-    [ASYNC_ITERATOR](): $AsyncIterator<T, T, void> {
-      let nextReadIndex = 0;
-      return createIterator(arg => {
-        if (arg !== undefined) {
-          throw new Error(
-            'Values cannot be passed to next() of AsyncIterables passed to Client Components.',
+
+  const iterable: $AsyncIterable<T, T, void> = ({}: any);
+  // $FlowFixMe[cannot-write]
+  iterable[ASYNC_ITERATOR] = (): $AsyncIterator<T, T, void> => {
+    let nextReadIndex = 0;
+    return createIterator(arg => {
+      if (arg !== undefined) {
+        throw new Error(
+          'Values cannot be passed to next() of AsyncIterables passed to Client Components.',
+        );
+      }
+      if (nextReadIndex === buffer.length) {
+        if (closed) {
+          // $FlowFixMe[invalid-constructor] Flow doesn't support functions as constructors
+          return new ReactPromise(
+            INITIALIZED,
+            {done: true, value: undefined},
+            null,
+            response,
           );
         }
-        if (nextReadIndex === buffer.length) {
-          if (closed) {
-            // $FlowFixMe[invalid-constructor] Flow doesn't support functions as constructors
-            return new ReactPromise(
-              INITIALIZED,
-              {done: true, value: undefined},
-              null,
-              response,
-            );
-          }
-          buffer[nextReadIndex] =
-            createPendingChunk<IteratorResult<T, T>>(response);
-        }
-        return buffer[nextReadIndex++];
-      });
-    },
+        buffer[nextReadIndex] =
+          createPendingChunk<IteratorResult<T, T>>(response);
+      }
+      return buffer[nextReadIndex++];
+    });
   };
+
   // TODO: If it's a single shot iterator we can optimize memory by cleaning up the buffer after
   // reading through the end, but currently we favor code size over this optimization.
   resolveStream(