Add tests for Subclassing the built-in Symbol Object

test/language/subclassing/Symbol/new-symbol-with-super-throws.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: 19.4.1
+description: Symbol subclass called with the new operator throws on super()
+info: >
+  19.4.1 The Symbol Constructor
+
+  ...
+  The Symbol constructor is not intended to be used with the new operator or to
+  be subclassed. It may be used as the value of an extends clause of a class
+  definition but a super call to the Symbol constructor will cause an exception.
+
+  19.4.1.1 Symbol ( [ description ] )
+
+  ...
+  1. If NewTarget is not undefined, throw a TypeError exception.
+---*/
+
+class S1 extends Symbol {}
+
+assert.throws(TypeError, function() {
+  new S1();
+});
+
+class S2 extends Symbol {
+  constructor() {
+    super();
+  }
+}
+
+assert.throws(TypeError, function() {
+  new S2();
+});
+

test/language/subclassing/Symbol/symbol-valid-as-extends-value.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: 19.4.1
+description: Symbol can be used as the value of an extends
+info: >
+  19.4.1 The Symbol Constructor
+
+  ...
+  The Symbol constructor is not intended to be used with the new operator or to
+  be subclassed. It may be used as the value of an extends clause of a class
+  definition but a super call to the Symbol constructor will cause an exception.
+  ...
+---*/
+
+class S extends Symbol {}