Test toString() behavior of invalid Dates

test/built-ins/Date/prototype/toString/invalid-date.js

@@ -0,0 +1,15 @@
+// Copyright (C) 2017 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-todatestring
+description: Invalid Dates are rendered as "Invalid Date"
+info: >
+  ToDateString ( tv )
+
+  ...
+  2. If tv is NaN, return "Invalid Date".
+  ...
+---*/
+
+assert.sameValue(new Date(NaN).toString(), "Invalid Date");