Introduce InternalPackedArray.

src/array.js

Index: src/array.js
diff --git a/src/array.js b/src/array.js
index 47f796d2b11e3a768b123fb369500cbd946937b8..9b0bfe19e6a25f25936901c5d206dffe54cad007 100644
--- a/src/array.js
+++ b/src/array.js
@@ -1559,6 +1559,12 @@ function SetUpArray() {
     "push", getFunction("push", ArrayPush),
     "splice", getFunction("splice", ArraySplice)
   ));
+
+  SetUpLockedPrototype(InternalPackedArray, $Array(), $Array(
+    "join", getFunction("join", ArrayJoin),
+    "pop", getFunction("pop", ArrayPop),
+    "push", getFunction("push", ArrayPush)
+  ));
 }
 
 SetUpArray();