Improve coverage for section 21: String (#712)

This changeset increases coverage for section 21, specifically "21.1 String
Objects".

* Add tests for "this" validation of String methods

* Add tests for ToNumber as used by String methods

* Add test for `length` prop of exotic String objs

* fixup! Add test for `length` prop of exotic String objs

test/built-ins/String/prototype/toLocaleUpperCase/this-value-not-obj-coercible.js

+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-string.prototype.tolocaleuppercase
+es6id: 21.1.3.21
+description: The "this" value must be object-coercible
+info: |
+  This function works exactly the same as toUpperCase except that its result is
+  intended to yield the correct result for the host environment's current
+  locale, rather than a locale-independent result.
+
+  21.1.3.26 String.prototype.toUpperCase
+
+  This function behaves in exactly the same way as
+  String.prototype.toLowerCase, except that code points are mapped to their
+  uppercase equivalents as specified in the Unicode Character Database.
+
+  21.1.3.24 String.prototype.toLowerCase
+
+  1. Let O be ? RequireObjectCoercible(this value).
+---*/
+
+var toLocaleUpperCase = String.prototype.toLocaleUpperCase;
+
+assert.sameValue(typeof toLocaleUpperCase, 'function');
+
+assert.throws(TypeError, function() {
+  toLocaleUpperCase.call(undefined);
+}, 'undefined');
+
+assert.throws(TypeError, function() {
+  toLocaleUpperCase.call(null);
+}, 'null');

test/built-ins/String/prototype/toLowerCase/this-value-not-obj-coercible.js

+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-string.prototype.tolowercase
+es6id: 21.1.3.22
+description: The "this" value must be object-coercible
+info: |
+  1. Let O be ? RequireObjectCoercible(this value).
+---*/
+
+var toLowerCase = String.prototype.toLowerCase;
+
+assert.sameValue(typeof toLowerCase, 'function');
+
+assert.throws(TypeError, function() {
+  toLowerCase.call(undefined);
+}, 'undefined');
+
+assert.throws(TypeError, function() {
+  toLowerCase.call(null);
+}, 'null');

test/built-ins/String/prototype/toUpperCase/this-value-not-obj-coercible.js

+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-string.prototype.touppercase
+es6id: 21.1.3.24
+description: The "this" value must be object-coercible
+info: |
+  This function behaves in exactly the same way as
+  String.prototype.toLowerCase, except that code points are mapped to their
+  uppercase equivalents as specified in the Unicode Character Database.
+
+  21.1.3.24 String.prototype.toLowerCase
+
+  1. Let O be ? RequireObjectCoercible(this value).
+---*/
+
+var toUpperCase = String.prototype.toUpperCase;
+
+assert.sameValue(typeof toUpperCase, 'function');
+
+assert.throws(TypeError, function() {
+  toUpperCase.call(undefined);
+}, 'undefined');
+
+assert.throws(TypeError, function() {
+  toUpperCase.call(null);
+}, 'null');