Fix ElementsKind handling of prototypes in Array.concat

test/mjsunit/regress/regress-crbug-412203.js

Index: test/mjsunit/regress/regress-crbug-412203.js
diff --git a/test/mjsunit/regress/regress-crbug-412203.js b/test/mjsunit/regress/regress-crbug-412203.js
new file mode 100644
index 0000000000000000000000000000000000000000..6a781308f75b2932ffb606f754f65911228728b9
--- /dev/null
+++ b/test/mjsunit/regress/regress-crbug-412203.js
@@ -0,0 +1,31 @@
+// Copyright 2014 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: --allow-natives-syntax
+
+var b = [];
+b[10000] = 1;
+// Required to reproduce the bug.
+assertTrue(%HasDictionaryElements(b));
+
+var a1 = [1.5];
+b.__proto__ = a1;
+assertEquals(1.5, ([].concat(b))[0]);
+
+var a2 = new Int32Array(2);
+a2[0] = 3;
+b.__proto__ = a2
+assertEquals(3, ([].concat(b))[0]);
+
+function foo(x, y) {
+  var a = [];
+  a[10000] = 1;
+  assertTrue(%HasDictionaryElements(a));
+
+  a.__proto__ = arguments;
+  var c = [].concat(a);
+  assertEquals(2, c[0]);
+  assertEquals(undefined, c[1]);
+}
+foo(2);