Always load the external runtime if one is provided

Author: sebmarkbage

packages/react-dom-bindings/src/server/ReactFizzConfigDOM.js

@@ -5089,15 +5089,10 @@ export function writePreambleStart(
   destination: Destination,
   resumableState: ResumableState,
   renderState: RenderState,
-  willFlushAllSegments: boolean,
   skipExpect?: boolean, // Used as an override by ReactFizzConfigMarkup
 ): void {
   // This function must be called exactly once on every request
-  if (
-    enableFizzExternalRuntime &&
-    !willFlushAllSegments &&
-    renderState.externalRuntimeScript
-  ) {
+  if (enableFizzExternalRuntime && renderState.externalRuntimeScript) {
     // If the root segment is incomplete due to suspended tasks
     // (e.g. willFlushAllSegments = false) and we are using data
     // streaming format, ensure the external runtime is sent.

packages/react-markup/src/ReactFizzConfigMarkup.js

@@ -208,14 +208,12 @@ export function writePreambleStart(
   destination: Destination,
   resumableState: ResumableState,
   renderState: RenderState,
-  willFlushAllSegments: boolean,
   skipExpect?: boolean, // Used as an override by ReactFizzConfigMarkup
 ): void {
   return writePreambleStartImpl(
     destination,
     resumableState,
     renderState,
-    willFlushAllSegments,
     true, // skipExpect
   );
 }

packages/react-server/src/ReactFizzServer.js

@@ -4835,14 +4835,7 @@ function flushPreamble(
   preambleSegments: Array<Array<Segment>>,
 ) {
   // The preamble is ready.
-  const willFlushAllSegments =
-    request.allPendingTasks === 0 && request.trackedPostpones === null;
-  writePreambleStart(
-    destination,
-    request.resumableState,
-    request.renderState,
-    willFlushAllSegments,
-  );
+  writePreambleStart(destination, request.resumableState, request.renderState);
   for (let i = 0; i < preambleSegments.length; i++) {
     const segments = preambleSegments[i];
     for (let j = 0; j < segments.length; j++) {