Support Iterables in Flight (#26313)

We support any super type of anything that we can serialize. Meaning
that as long as the Type that's passed through is less precise, it means
that we can encoded it as any subtype and therefore the incoming type
doesn't have to be the subtype in that case. Basically, as long as
you're only passing through an `Iterable<T>` in TypeScript, then you can
pass any `Iterable<T>` and we'll treat it as an array.

For example we support Promises *and* Thenables but both are encoded as
Promises.

We support Arrays and since Arrays are also Iterables, we can support
Iterables.

For @wongmjane

packages/react-client/src/__tests__/ReactFlight-test.js

@@ -169,6 +169,34 @@ describe('ReactFlight', () => {
     expect(ReactNoop).toMatchRenderedOutput(<span>Hello, Seb Smith</span>);
   });
 
+  it('can render an iterable as an array', async () => {
+    function ItemListClient(props) {
+      return <span>{props.items}</span>;
+    }
+    const ItemList = clientReference(ItemListClient);
+
+    function Items() {
+      const iterable = {
+        [Symbol.iterator]: function* () {
+          yield 'A';
+          yield 'B';
+          yield 'C';
+        },
+      };
+      return <ItemList items={iterable} />;
+    }
+
+    const model = <Items />;
+
+    const transport = ReactNoopFlightServer.render(model);
+
+    await act(async () => {
+      ReactNoop.render(await ReactNoopFlightClient.read(transport));
+    });
+
+    expect(ReactNoop).toMatchRenderedOutput(<span>ABC</span>);
+  });
+
   it('can render a lazy component as a shared component on the server', async () => {
     function SharedComponent({text}) {
       return (

packages/react-server/src/ReactFlightServer.js

@@ -73,6 +73,7 @@ import {
 } from './ReactFlightNewContext';
 
 import {
+  getIteratorFn,
   REACT_ELEMENT_TYPE,
   REACT_FORWARD_REF_TYPE,
   REACT_FRAGMENT_TYPE,
@@ -1059,6 +1060,12 @@ export function resolveModelToJSON(
       }
       return (undefined: any);
     }
+    if (!isArray(value)) {
+      const iteratorFn = getIteratorFn(value);
+      if (iteratorFn) {
+        return Array.from((value: any));
+      }
+    }
 
     if (__DEV__) {
       if (value !== null && !isArray(value)) {