[api] Prototype WeakRef implementation

src/js/macros.py

Index: src/js/macros.py
diff --git a/src/js/macros.py b/src/js/macros.py
index 3508cc82701248be3b9cabab99380aadf91959bc..81bdb08165742fa7540204202f8072b3442a0303 100644
--- a/src/js/macros.py
+++ b/src/js/macros.py
@@ -70,6 +70,7 @@ macro IS_TYPEDARRAY(arg)        = (%_IsTypedArray(arg));
 macro IS_UNDEFINED(arg)         = (arg === (void 0));
 macro IS_WEAKMAP(arg)           = (%_ClassOf(arg) === 'WeakMap');
 macro IS_WEAKSET(arg)           = (%_ClassOf(arg) === 'WeakSet');
+macro IS_WEAKREF(arg)           = (%_ClassOf(arg) === 'WeakRef');
 
 # Macro for ES queries of the type: "Type(O) is Object."
 macro IS_RECEIVER(arg) = (%_IsJSReceiver(arg));