Swap the primary renderer flag in the legacy stream config

This allows the legacy implementation to co-exist with the new
implementation.

But primarily it allows Fiber and the legacy renderToString to co-exist.

packages/react-dom/src/server/ReactDOMLegacyServerStreamConfig.js

@@ -16,6 +16,8 @@ export type Destination = {
 export type PrecomputedChunk = string;
 export type Chunk = string;
 
+export const isPrimaryStreamConfig = false;
+
 export function scheduleWork(callback: () => void) {
   callback();
 }

packages/react-dom/src/server/ReactDOMServerFormatConfig.js

@@ -23,6 +23,7 @@ import {
   writeChunk,
   stringToChunk,
   stringToPrecomputedChunk,
+  isPrimaryStreamConfig,
 } from 'react-server/src/ReactServerStreamConfig';
 
 import {
@@ -50,7 +51,7 @@ import isArray from 'shared/isArray';
 
 // Used to distinguish these contexts from ones used in other renderers.
 // E.g. this can be used to distinguish legacy renderers from this modern one.
-export const isPrimaryRenderer = true;
+export const isPrimaryRenderer = isPrimaryStreamConfig;
 
 // Per response, global state that is not contextual to the rendering subtree.
 export type ResponseState = {

packages/react-server/src/ReactServerStreamConfigBrowser.js

@@ -12,6 +12,8 @@ export type Destination = ReadableStreamController;
 export type PrecomputedChunk = Uint8Array;
 export type Chunk = Uint8Array;
 
+export const isPrimaryStreamConfig = true;
+
 export function scheduleWork(callback: () => void) {
   callback();
 }

packages/react-server/src/ReactServerStreamConfigNode.js

@@ -21,6 +21,8 @@ export type Destination = Writable & MightBeFlushable;
 export type PrecomputedChunk = Uint8Array;
 export type Chunk = string;
 
+export const isPrimaryStreamConfig = true;
+
 export function scheduleWork(callback: () => void) {
   setImmediate(callback);
 }