Add tests for Subclassing the built-in Date Objects

test/language/subclassing/Date/regular-subclassing.js

+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+es6id: 20.3.2
+description: Subclassing the String object
+info: >
+  20.3.2 The Date Constructor
+
+  ...
+
+  The Date constructor is a single function whose behaviour is overloaded based
+  upon the number and types of its arguments.
+
+  The Date constructor is designed to be subclassable. It may be used as the
+  value of an extends clause of a class definition. Subclass constructors that
+  intend to inherit the specified Date behaviour must include a super call to
+  the Date constructor to create and initialize the subclass instance with a
+  [[DateValue]] internal slot.
+---*/
+
+class D extends Date {}
+
+var d1 = new D(1859, '10', 24);
+assert.sameValue(d1.getUTCFullYear(), 1859);
+assert.sameValue(d1.getUTCMonth(), 10);
+assert.sameValue(d1.getUTCDate(), 24);
+
+var d2 = new D(-3474558000000);
+assert.sameValue(d2.getUTCFullYear(), 1859);
+assert.sameValue(d2.getUTCMonth(), 10);
+assert.sameValue(d2.getUTCDate(), 24);
+
+var d3 = new D();
+var d4 = new Date();
+assert.sameValue(d3.getUTCFullYear(), d4.getUTCFullYear());
+assert.sameValue(d3.getUTCMonth(), d4.getUTCMonth());
+assert.sameValue(d3.getUTCDate(), d4.getUTCDate());

test/language/subclassing/Date/super-must-be-called.js

+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+es6id: 20.3.2
+description: Super need to be called to initialize internals
+info: >
+  20.3.2 The Date Constructor
+
+  ...
+
+  The Date constructor is a single function whose behaviour is overloaded based
+  upon the number and types of its arguments.
+
+  The Date constructor is designed to be subclassable. It may be used as the
+  value of an extends clause of a class definition. Subclass constructors that
+  intend to inherit the specified Date behaviour must include a super call to
+  the Date constructor to create and initialize the subclass instance with a
+  [[DateValue]] internal slot.
+---*/
+
+class D extends Date {
+  constructor() {}
+}
+
+assert.throws(ReferenceError, function() {
+  new D();
+});
+
+class D2 extends Date {
+  constructor() {
+    super();
+  }
+}
+
+new D2();