[Flight] Serialize deduped elements by direct reference even if they …

…suspend (facebook#28283)

In facebook#28123 I switched these to be lazy references. However that creates a
lazy wrapper even if they're synchronously available. We try to as much
as possible preserve the original data structure in these cases.

E.g. here in the dev outlining I only use a lazy wrapper if it didn't
complete synchronously:
https://github.com/facebook/react/pull/28272/files#diff-d4c9c509922b3671d3ecce4e051df66dd5c3d38ff913c7a7fe94abc3ba2ed72eR638

Unfortunately we don't have a data structure that tracks the status of
each emitted row. We could store the task in the map but then they
couldn't be GC:ed as they complete. We could maybe store the status of
each element but seems so heavy.

For now I just went back to direct reference which might be an issue
since it can suspend something higher up when deduped.

packages/react-server/src/ReactFlightServer.js

@@ -1192,12 +1192,16 @@ function renderModelDestructive(
             // but that is able to reuse the same task if we're already in one but then that
             // will be a lazy future value rather than guaranteed to exist but maybe that's good.
             const newId = outlineModel(request, (value: any));
-            return serializeLazyID(newId);
+            return serializeByValueID(newId);
           } else {
             // We've already emitted this as an outlined object, so we can refer to that by its
-            // existing ID. We use a lazy reference since, unlike plain objects, elements might
-            // suspend so it might not have emitted yet even if we have the ID for it.
-            return serializeLazyID(existingId);
+            // existing ID. TODO: We should use a lazy reference since, unlike plain objects,
+            // elements might suspend so it might not have emitted yet even if we have the ID for
+            // it. However, this creates an extra wrapper when it's not needed. We should really
+            // detect whether this already was emitted and synchronously available. In that
+            // case we can refer to it synchronously and only make it lazy otherwise.
+            // We currently don't have a data structure that lets us see that though.
+            return serializeByValueID(existingId);
           }
         } else {
           // This is the first time we've seen this object. We may never see it again