LibJS: Add the WeakMap built-in object

Userland/Libraries/LibJS/Runtime/WeakMap.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/HashMap.h>
+#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/Object.h>
+#include <LibJS/Runtime/WeakContainer.h>
+
+namespace JS {
+
+class WeakMap final
+    : public Object
+    , public WeakContainer {
+    JS_OBJECT(WeakMap, Object);
+
+public:
+    static WeakMap* create(GlobalObject&);
+
+    explicit WeakMap(Object& prototype);
+    virtual ~WeakMap() override;
+
+    HashMap<Cell*, Value> const& values() const { return m_values; };
+    HashMap<Cell*, Value>& values() { return m_values; };
+
+    virtual void remove_sweeped_cells(Badge<Heap>, Vector<Cell*>&) override;
+
+private:
+    HashMap<Cell*, Value> m_values; // This stores Cell pointers instead of Object pointers to aide with sweeping
+};
+
+}