Use gate pragma instead of if (__EXPERIMENTAL__) (#19722)

* Use gate pragma instead of if (__EXPERIMENTAL__)

* Fix stream error handling in tests

Added an error listener so that the tests fail within their Jest scope,
instead of crashing the whole process.

packages/react-dom/src/__tests__/utils/ReactDOMServerIntegrationTestUtils.js

@@ -144,9 +144,11 @@ module.exports = function(initModules) {
   async function renderIntoStream(reactElement, errorCount = 0) {
     return await expectErrors(
       () =>
-        new Promise(resolve => {
+        new Promise((resolve, reject) => {
           const writable = new DrainWritable();
-          ReactDOMServer.renderToNodeStream(reactElement).pipe(writable);
+          const s = ReactDOMServer.renderToNodeStream(reactElement);
+          s.on('error', e => reject(e));
+          s.pipe(writable);
           writable.on('finish', () => resolve(writable.buffer));
         }),
       errorCount,