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diff --git a/js/src/builtin/streams/StreamController.h b/js/src/builtin/streams/StreamController.h
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+++ b/js/src/builtin/streams/StreamController.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Base class for readable and writable stream controllers. */
+
+#ifndef builtin_streams_StreamController_h
+#define builtin_streams_StreamController_h
+
+#include "js/Value.h"         // JS::Value, JS::NumberValue
+#include "vm/JSObject.h"      // JSObject
+#include "vm/List.h"          // js::ListObject
+#include "vm/NativeObject.h"  // js::NativeObject
+
+namespace js {
+
+/**
+ * Common base class of both readable and writable stream controllers.
+ */
+class StreamController : public NativeObject {
+ public:
+  /**
+   * Memory layout for stream controllers.
+   *
+   * Both ReadableStreamDefaultController and ReadableByteStreamController
+   * are queue containers and must have these slots at identical offsets.
+   *
+   * The queue is guaranteed to be in the same compartment as the container,
+   * but might contain wrappers for objects from other compartments.
+   */
+  enum Slots { Slot_Queue, Slot_TotalSize, SlotCount };
+
+  ListObject* queue() const {
+    return &getFixedSlot(Slot_Queue).toObject().as<ListObject>();
+  }
+  double queueTotalSize() const {
+    return getFixedSlot(Slot_TotalSize).toNumber();
+  }
+  void setQueueTotalSize(double size) {
+    setFixedSlot(Slot_TotalSize, JS::NumberValue(size));
+  }
+};
+
+}  // namespace js
+
+template <>
+inline bool JSObject::is<js::StreamController>() const;
+
+#endif  // builtin_streams_StreamController_h