Add a way to create Server Reference Proxies on the client

This lets the client bundle encode Server References without them first
being passed from an RSC payload. Like if you just import "use server"
from the client.

In the future we could expand this to allow .bind() too.

packages/react-client/src/ReactFlightReplyClient.js

@@ -9,7 +9,10 @@
 
 import type {Thenable} from 'shared/ReactTypes';
 
-import {knownServerReferences} from './ReactFlightServerReferenceRegistry';
+import {
+  knownServerReferences,
+  createServerReference,
+} from './ReactFlightServerReferenceRegistry';
 
 import {
   REACT_ELEMENT_TYPE,
@@ -312,3 +315,5 @@ export function processReply(
     }
   }
 }
+
+export {createServerReference};

packages/react-client/src/ReactFlightServerReferenceRegistry.js

@@ -9,9 +9,24 @@
 
 import type {Thenable} from 'shared/ReactTypes';
 
+export type CallServerCallback = <A, T>(id: any, args: A) => Promise<T>;
+
 type ServerReferenceId = any;
 
 export const knownServerReferences: WeakMap<
   Function,
   {id: ServerReferenceId, bound: null | Thenable<Array<any>>},
 > = new WeakMap();
+
+export function createServerReference<A: Iterable<any>, T>(
+  id: ServerReferenceId,
+  callServer: CallServerCallback,
+): (...A) => Promise<T> {
+  const proxy = function (): Promise<T> {
+    // $FlowFixMe[method-unbinding]
+    const args = Array.prototype.slice.call(arguments);
+    return callServer(id, args);
+  };
+  knownServerReferences.set(proxy, {id: id, bound: null});
+  return proxy;
+}

packages/react-server-dom-webpack/src/ReactFlightDOMClientBrowser.js

@@ -22,7 +22,10 @@ import {
   close,
 } from 'react-client/src/ReactFlightClientStream';
 
-import {processReply} from 'react-client/src/ReactFlightReplyClient';
+import {
+  processReply,
+  createServerReference,
+} from 'react-client/src/ReactFlightReplyClient';
 
 type CallServerCallback = <A, T>(string, args: A) => Promise<T>;
 
@@ -125,4 +128,10 @@ function encodeReply(
   });
 }
 
-export {createFromXHR, createFromFetch, createFromReadableStream, encodeReply};
+export {
+  createFromXHR,
+  createFromFetch,
+  createFromReadableStream,
+  encodeReply,
+  createServerReference,
+};

packages/react-server-dom-webpack/src/__tests__/ReactFlightDOMBrowser-test.js

@@ -893,6 +893,70 @@ describe('ReactFlightDOMBrowser', () => {
     expect(result).toBe('Hello Split');
   });
 
+  it('can pass a server function by importing from client back to server', async () => {
+    function greet(transform, text) {
+      return 'Hello ' + transform(text);
+    }
+
+    function upper(text) {
+      return text.toUpperCase();
+    }
+
+    const ServerModuleA = serverExports({
+      greet,
+    });
+    const ServerModuleB = serverExports({
+      upper,
+    });
+
+    let actionProxy;
+
+    // This is a Proxy representing ServerModuleB in the Client bundle.
+    const ServerModuleBImportedOnClient = {
+      upper: ReactServerDOMClient.createServerReference(
+        ServerModuleB.upper.$$id,
+        async function(ref, args) {
+          const body = await ReactServerDOMClient.encodeReply(args);
+          return callServer(ref, body);
+        },
+      ),
+    };
+
+    function Client({action}) {
+      // Client side pass a Server Reference into an action.
+      actionProxy = text => action(ServerModuleBImportedOnClient.upper, text);
+      return 'Click Me';
+    }
+
+    const ClientRef = clientExports(Client);
+
+    const stream = ReactServerDOMServer.renderToReadableStream(
+      <ClientRef action={ServerModuleA.greet} />,
+      webpackMap,
+    );
+
+    const response = ReactServerDOMClient.createFromReadableStream(stream, {
+      async callServer(ref, args) {
+        const body = await ReactServerDOMClient.encodeReply(args);
+        return callServer(ref, body);
+      },
+    });
+
+    function App() {
+      return use(response);
+    }
+
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<App />);
+    });
+    expect(container.innerHTML).toBe('Click Me');
+
+    const result = await actionProxy('hi');
+    expect(result).toBe('Hello HI');
+  });
+
   it('can bind arguments to a server reference', async () => {
     let actionProxy;