Fix string HTML methods to call ToString

test/mjsunit/es6/string-html.js

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Tests taken from:
 // http://mathias.html5.org/tests/javascript/string/
 
 assertEquals('_'.anchor('b'), '<a name="b">_</a>');
 assertEquals('<'.anchor('<'), '<a name="<"><</a>');
 assertEquals('_'.anchor(0x2A), '<a name="42">_</a>');
@@ skipping 139 matching lines @@
 assertEquals('_'.sup(), '<sup>_</sup>');
 assertEquals('<'.sup(), '<sup><</sup>');
 assertEquals(String.prototype.sup.call(0x2A), '<sup>42</sup>');
 assertThrows(function() {
   String.prototype.sup.call(undefined);
 }, TypeError);
 assertThrows(function() {
   String.prototype.sup.call(null);
 }, TypeError);
 assertEquals(String.prototype.sup.length, 0);
+
+
+(function TestToString() {
+  var calls = 0;
+  var obj = {
+    toString() {
+      calls++;
+      return 'abc';
+    },
+    valueOf() {
+      assertUnreachable();
+    }
+  };
+
+  var methodNames = [
+    'anchor',
+    'big',
+    'blink',
+    'bold',
+    'fixed',
+    'fontcolor',
+    'fontsize',
+    'italics',
+    'link',
+    'small',
+    'strike',
+    'sub',
+    'sup',
+  ];
+  for (var name of methodNames) {
+    calls = 0;
+    String.prototype[name].call(obj);
+    assertEquals(1, calls);
+  }
+})();