Add tests for Subclassing the built-in Number Objects

test/language/subclassing/Number/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: 20.1.1
+description: Subclassing the Number object
+info: >
+  20.1.1 The Number Constructor
+
+  ...
+
+  The Number 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 Number behaviour must include a super call to
+  the Number constructor to create and initialize the subclass instance with a
+  [[NumberData]] internal slot.
+---*/
+
+class N extends Number {}
+
+var n = new N(42);
+
+assert.sameValue(n.toFixed(2), '42.00');
+assert.sameValue(n.toExponential(2), '4.20e+1');

test/language/subclassing/Number/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: 20.1.1
+description: Super need to be called to initialize internals
+info: >
+  20.1.1 The Number Constructor
+
+  ...
+
+  The Number 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 Number behaviour must include a super call to
+  the Number constructor to create and initialize the subclass instance with a
+  [[NumberData]] internal slot.
+---*/
+
+class N extends Number {
+  constructor() {}
+}
+
+assert.throws(ReferenceError, function() {
+  new N();
+});
+
+class N2 extends Number {
+  constructor() {
+    super();
+  }
+}
+
+new N2();