Promote ASYNC_ITERATOR symbol to React Symbols (#28851)

So that when we end up referring to it in more places, it's only one.

We don't do this same pattern for regular `Symbol.iterator` because we
also support the string `"@@iterator"` for backwards compatibility.

Author: sebmarkbage

packages/react-client/src/ReactFlightClient.js

@@ -65,6 +65,7 @@ import {
   REACT_LAZY_TYPE,
   REACT_ELEMENT_TYPE,
   REACT_POSTPONE_TYPE,
+  ASYNC_ITERATOR,
 } from 'shared/ReactSymbols';
 
 export type {CallServerCallback, EncodeFormActionCallback};
@@ -1245,8 +1246,6 @@ function startReadableStream<T>(
   resolveStream(response, id, stream, flightController);
 }
 
-const ASYNC_ITERATOR = Symbol.asyncIterator;
-
 function asyncIterator(this: $AsyncIterator<any, any, void>) {
   // Self referencing iterator.
   return this;

packages/react-server/src/ReactFlightServer.js

@@ -99,6 +99,7 @@ import {
   REACT_LAZY_TYPE,
   REACT_MEMO_TYPE,
   REACT_POSTPONE_TYPE,
+  ASYNC_ITERATOR,
 } from 'shared/ReactSymbols';
 
 import {
@@ -201,8 +202,6 @@ if (
 
 const ObjectPrototype = Object.prototype;
 
-const ASYNC_ITERATOR = Symbol.asyncIterator;
-
 type JSONValue =
   | string
   | boolean

packages/shared/ReactSymbols.js

@@ -60,3 +60,5 @@ export function getIteratorFn(maybeIterable: ?any): ?() => ?Iterator<any> {
   }
   return null;
 }
+
+export const ASYNC_ITERATOR = Symbol.asyncIterator;