[Flight] response.readRoot() -> use(response) (#25267)

* [Flight] Move from suspensey readRoot() to use(thenable)

* Update noop tests

These are no longer sync so they need some more significant updating.

Some of these tests are written in a non-idiomatic form too which is not
great.

* Update Relay tests

I kept these as sync for now and just assume a sync Promise.

* Updated the main tests

* Gate tests

* We need to cast through any because Thenable doesn't support unknown strings

fixtures/flight-browser/index.html

@@ -83,7 +83,7 @@ <h1>Flight Example</h1>
       }
 
       function Shell({ data }) {
-        let model = data.readRoot();
+        let model = React.experimental_use(data);
         return <div>
           <Suspense fallback="...">
             <h1>{model.title}</h1>

fixtures/flight/src/index.js

@@ -6,7 +6,7 @@ import ReactServerDOMReader from 'react-server-dom-webpack';
 let data = ReactServerDOMReader.createFromFetch(fetch('http://localhost:3001'));
 
 function Content() {
-  return data.readRoot();
+  return React.experimental_use(data);
 }
 
 ReactDOM.render(

packages/react-client/src/ReactFlightClient.js

@@ -147,7 +147,6 @@ Chunk.prototype.then = function<T>(
 export type ResponseBase = {
   _bundlerConfig: BundlerConfig,
   _chunks: Map<number, SomeChunk<any>>,
-  readRoot<T>(): T,
   ...
 };
 
@@ -177,10 +176,9 @@ function readChunk<T>(chunk: SomeChunk<T>): T {
   }
 }
 
-function readRoot<T>(): T {
-  const response: Response = this;
+export function getRoot<T>(response: Response): Thenable<T> {
   const chunk = getChunk(response, 0);
-  return readChunk(chunk);
+  return (chunk: any);
 }
 
 function createPendingChunk<T>(response: Response): PendingChunk<T> {
@@ -541,7 +539,6 @@ export function createResponse(bundlerConfig: BundlerConfig): ResponseBase {
   const response = {
     _bundlerConfig: bundlerConfig,
     _chunks: chunks,
-    readRoot: readRoot,
   };
   return response;
 }

packages/react-client/src/ReactFlightClientStream.js

@@ -137,4 +137,4 @@ export function createResponse(bundlerConfig: BundlerConfig): Response {
   return response;
 }
 
-export {reportGlobalError, close} from './ReactFlightClient';
+export {reportGlobalError, getRoot, close} from './ReactFlightClient';