Add tests for Subclassing the built-in RegExp Objects

test/language/subclassing/RegExp/lastIndex.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: 21.2.6.1
+description: Instances has the own property lastIndex
+info: >
+  21.2.6.1 lastIndex
+
+  The value of the lastIndex property specifies the String index at which to
+  start the next match. It is coerced to an integer when used (see 21.2.5.2.2).
+  This property shall have the attributes { [[Writable]]: true, [[Enumerable]]:
+  false, [[Configurable]]: false }.
+includes: [propertyHelper.js]
+---*/
+
+class RE extends RegExp {}
+
+var re = new RE('39?');
+
+re.exec('TC39');
+
+assert.sameValue(re.lastIndex, 0);
+
+verifyWritable(re, 'lastIndex');
+verifyNotEnumerable(re, 'lastIndex');
+verifyNotConfigurable(re, 'lastIndex');
\ No newline at end of file

test/language/subclassing/RegExp/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: 21.2.3
+description: Subclassing the RegExp object
+info: >
+  21.2.3 The RegExp Constructor
+
+  ...
+
+  The RegExp 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 RegExp behaviour must include a super call to
+  the RegExp constructor to create and initialize subclass instances with the
+  necessary internal slots.
+---*/
+
+class RE extends RegExp {}
+
+var re = new RE(39);
+
+assert.sameValue(re.test('TC39'), true);
+assert.sameValue(re.test('42'), false);

test/language/subclassing/RegExp/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: 21.2.3
+description: Super need to be called to initialize internals
+info: >
+  21.2.3 The RegExp Constructor
+
+  ...
+
+  The RegExp 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 RegExp behaviour must include a super call to
+  the RegExp constructor to create and initialize subclass instances with the
+  necessary internal slots.
+---*/
+
+class RE1 extends RegExp {
+  constructor() {}
+}
+
+assert.throws(ReferenceError, function() {
+  new RE1();
+});
+
+class RE2 extends RegExp {
+  constructor() {
+    super();
+  }
+}
+
+new RE2();