Object.prototype.toString must reflect mutated @@toStringTag values for primitives

test/mjsunit/regress/regress-5780.js

Index: test/mjsunit/regress/regress-5780.js
diff --git a/test/mjsunit/regress/regress-5780.js b/test/mjsunit/regress/regress-5780.js
new file mode 100644
index 0000000000000000000000000000000000000000..5a6f1711c15d7ec505ca66ef5697d856b8e12950
--- /dev/null
+++ b/test/mjsunit/regress/regress-5780.js
@@ -0,0 +1,16 @@
+// Copyright 2016 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.
+
+function testMutatedPrimitiveToStringTag(primitive) {
+  Object.defineProperty(
+    primitive.__proto__, Symbol.toStringTag,
+    {value: "bogus", configurable: true, writable: false, enumerable: false});
+  assertEquals("[object bogus]", Object.prototype.toString.call(primitive));
+}
+
+testMutatedPrimitiveToStringTag('');
+testMutatedPrimitiveToStringTag(true);
+testMutatedPrimitiveToStringTag(42);
+testMutatedPrimitiveToStringTag(42.42);
+testMutatedPrimitiveToStringTag(Symbol());