[builtins] TurboFan version of Array.prototype.forEach including fast path for FAST_ELEMENTS

test/mjsunit/proto-elements-add-during-foreach.js

Index: test/mjsunit/proto-elements-add-during-foreach.js
diff --git a/test/mjsunit/proto-elements-add-during-foreach.js b/test/mjsunit/proto-elements-add-during-foreach.js
new file mode 100644
index 0000000000000000000000000000000000000000..a99e8070d0fd07d4ecf6c3f311726dc9360f6f6a
--- /dev/null
+++ b/test/mjsunit/proto-elements-add-during-foreach.js
@@ -0,0 +1,12 @@
+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --enable-fast-array-builtins
+
+var a = [0,1,2,,,,7];
+var proto = {}
+a.__proto__ = proto;
+var visits = 0;
+Array.prototype.forEach.call(a, (v,i,o) => { ++visits; proto[4] = 4; });
+assertEquals(5, visits);