--- a/js/src/builtin/Stream.cpp
+++ b/js/src/builtin/Stream.cpp
@@ -574,30 +574,30 @@ ReadableStream::createDefaultStream(JSCo
     if (!controller) {
         return nullptr;
     }
     stream->setController(controller);
     return stream;
 }
 
 static MOZ_MUST_USE ReadableByteStreamController*
-CreateReadableByteStreamController(JSContext* cx, Handle<ReadableStream*> stream,
-                                   void* underlyingSource);
+CreateExternalReadableByteStreamController(JSContext* cx, Handle<ReadableStream*> stream,
+                                           void* underlyingSource);
 
 ReadableStream*
 ReadableStream::createExternalSourceStream(JSContext* cx, void* underlyingSource,
                                            uint8_t flags, HandleObject proto /* = nullptr */)
 {
     Rooted<ReadableStream*> stream(cx, createStream(cx, proto));
     if (!stream) {
         return nullptr;
     }
 
     Rooted<ReadableStreamController*> controller(cx);
-    controller = CreateReadableByteStreamController(cx, stream, underlyingSource);
+    controller = CreateExternalReadableByteStreamController(cx, stream, underlyingSource);
     if (!controller) {
         return nullptr;
     }
 
     stream->setController(controller);
     controller->setEmbeddingFlags(flags);
 
     return stream;
@@ -3072,16 +3072,18 @@ ReadableStreamControllerGetDesiredSizeUn
 
     // Step 5: Return controller.[[strategyHWM]] − controller.[[queueTotalSize]].
     return controller->strategyHWM() - controller->queueTotalSize();
 }
 
 
 /*** 3.10. Class ReadableByteStreamController ****************************************************/
 
+#if 0 // disable user-defined byte streams
+
 /**
  * Streams spec, 3.10.3 new ReadableByteStreamController ( stream, underlyingSource,
  *                                                         highWaterMark )
  * Steps 3 - 16.
  *
  * Note: can NOT operate on unwrapped ReadableStream instances from
  * another compartment: ReadableStream controllers must be created in the same
  * compartment as the stream.
@@ -3184,33 +3186,35 @@ CreateReadableByteStreamController(JSCon
 
     if (!JS::AddPromiseReactions(cx, startPromise, onStartFulfilled, onStartRejected)) {
         return nullptr;
     }
 
     return controller;
 }
 
+#endif  // user-defined byte streams
+
 // Streams spec, 3.10.3.
 // new ReadableByteStreamController ( stream, underlyingByteSource,
 //                                    highWaterMark )
 bool
 ReadableByteStreamController::constructor(JSContext* cx, unsigned argc, Value* vp)
 {
     // Step 1: Throw a TypeError exception.
     JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
                               JSMSG_BOGUS_CONSTRUCTOR, "ReadableByteStreamController");
     return false;
 }
 
 // Version of the ReadableByteStreamConstructor that's specialized for
 // handling external, embedding-provided, underlying sources.
 static MOZ_MUST_USE ReadableByteStreamController*
-CreateReadableByteStreamController(JSContext* cx, Handle<ReadableStream*> stream,
-                                   void* underlyingSource)
+CreateExternalReadableByteStreamController(JSContext* cx, Handle<ReadableStream*> stream,
+                                           void* underlyingSource)
 {
     Rooted<ReadableByteStreamController*> controller(cx);
     controller = NewBuiltinClassInstance<ReadableByteStreamController>(cx);
     if (!controller) {
         return nullptr;
     }
 
     // Step 3: Set this.[[controlledReadableStream]] to stream.