Add tests for Subclassing the built-in String Objects

test/language/subclassing/String/length.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: 21.1.4
+description: Instances has the own property length
+info: >
+  21.1.4 Properties of String Instances
+
+  ...
+
+  String instances have a length property, and a set of enumerable properties
+  with integer indexed names.
+includes: [propertyHelper.js]
+---*/
+
+class S extends String {}
+
+var s1 = new S();
+assert.sameValue(s1.length, 0);
+
+verifyNotWritable(s1, 'length');
+verifyNotEnumerable(s1, 'length');
+verifyNotConfigurable(s1, 'length');
+
+var s2 = new S('test262');
+assert.sameValue(s2.length, 7);
+
+verifyNotWritable(s2, 'length');
+verifyNotEnumerable(s2, 'length');
+verifyNotConfigurable(s2, 'length');
\ No newline at end of file

test/language/subclassing/String/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: 21.1.1
+description: Subclassing the String object
+info: >
+  21.1.1 The String Constructor
+
+  ...
+  The String 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 String behaviour must include a super call to
+  the String constructor to create and initialize the subclass instance with a
+  [[StringData]] internal slot.
+---*/
+
+class S extends String {}
+
+var s = new S(' test262 ');
+
+assert.sameValue(s.trim(), 'test262');
+

test/language/subclassing/String/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: 21.1.1
+description: Super need to be called to initialize internals
+info: >
+  21.1.1 The String Constructor
+
+  ...
+  The String 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 String behaviour must include a super call to
+  the String constructor to create and initialize the subclass instance with a
+  [[StringData]] internal slot.
+---*/
+
+class S1 extends String {
+  constructor() {}
+}
+
+assert.throws(ReferenceError, function() {
+  new S1();
+});
+
+class S2 extends String {
+  constructor() {
+    super();
+  }
+}
+
+new S2();