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.

DiffTrain build for [c0cf7c6](c0cf7c6)

compiled/facebook-www/REVISION

@@ -1 +1 @@
-7909d8eabb7a702618f51e16a351df41aa8da88e
+c0cf7c696cf454b49b35d8dae01ab111739dab46

compiled/facebook-www/ReactFlightDOMClient-dev.modern.js

@@ -303,6 +303,7 @@ if (__DEV__) {
     // The Symbol used to tag the ReactElement-like types.
     var REACT_ELEMENT_TYPE = Symbol.for("react.element");
     var REACT_LAZY_TYPE = Symbol.for("react.lazy");
+    var ASYNC_ITERATOR = Symbol.asyncIterator;
 
     function readTemporaryReference(set, id) {
       if (id < 0 || id >= set.length) {
@@ -1172,8 +1173,6 @@ if (__DEV__) {
       }
     }
 
-    var ASYNC_ITERATOR = Symbol.asyncIterator;
-
     function resolveErrorDev(response, id, digest, message, stack) {
       var error = new Error(
         message ||