Fix string HTML methods to call ToString

test/mjsunit/es6/string-html.js

Index: test/mjsunit/es6/string-html.js
diff --git a/test/mjsunit/es6/string-html.js b/test/mjsunit/es6/string-html.js
index 4f3feb56dd1f5e7e10b1f3f9a5e153b450b84f15..1c63221c9f7ec6b0c8949628f8ebd4bffadffe9d 100644
--- a/test/mjsunit/es6/string-html.js
+++ b/test/mjsunit/es6/string-html.js
@@ -157,3 +157,38 @@ 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);
+  }
+})();