Bug 1209471 - Fix group used for Array.concat result, r=jandem. a=al

--HG--
extra : source : b17006f337c4b1bfbc17b10d2e73a98bfe980217

js/src/jsarray.cpp

@@ -2639,7 +2639,7 @@ js::array_concat(JSContext* cx, unsigned argc, Value* vp)
         argc--;
         p++;
     } else {
-        narr = NewDenseEmptyArray(cx);
+        narr = NewFullyAllocatedArrayTryReuseGroup(cx, aobj, 0);
         if (!narr)
             return false;
         args.rval().setObject(*narr);