[Flight] Don't block on debug channel if it's not wired up (facebook#33757)

React Elements reference debug data (their stack and owner) in the debug
channel. If the debug channel isn't wired up this can block the client
from resolving.

We can infer that if there's no debug channel wired up and the reference
wasn't emitted before the element, then it's probably because it's in
the debug channel. So we can skip it.

This should also apply to debug chunks but they're not yet blocking
until facebook#33665 lands.

Author: sebmarkbage

packages/react-client/src/ReactFlightClient.js

@@ -1353,6 +1353,26 @@ function waitForReference<T>(
   map: (response: Response, model: any, parentObject: Object, key: string) => T,
   path: Array<string>,
 ): T {
+  if (
+    __DEV__ &&
+    // TODO: This should check for the existence of the "readable" side, not the "writable".
+    response._debugChannel === undefined
+  ) {
+    if (
+      referencedChunk.status === PENDING &&
+      parentObject[0] === REACT_ELEMENT_TYPE &&
+      (key === '4' || key === '5')
+    ) {
+      // If the parent object is an unparsed React element tuple, and this is a reference
+      // to the owner or debug stack. Then we expect the chunk to have been emitted earlier
+      // in the stream. It might be blocked on other things but chunk should no longer be pending.
+      // If it's still pending that suggests that it was referencing an object in the debug
+      // channel, but no debug channel was wired up so it's missing. In this case we can just
+      // drop the debug info instead of halting the whole stream.
+      return (null: any);
+    }
+  }
+
   let handler: InitializationHandler;
   if (initializingHandler) {
     handler = initializingHandler;