Avoid using Function#bind when copying a method's toString and `v…

…alueOf` methods to its wrapped version in class creation.

The previous commit fixes the issue in our own polyfill that was causing this error, but we're changing this anyway because people might be relying on the MDC polyfill in the wild.

src/prototype/lang/class.js

@@ -167,9 +167,22 @@ var Class = (function() {
         value = (function(m) {
           return function() { return ancestor[m].apply(this, arguments); };
         })(property).wrap(method);
-
-        value.valueOf = method.valueOf.bind(method);
-        value.toString = method.toString.bind(method);
+
+        // We used to use `bind` to ensure that `toString` and `valueOf`
+        // methods were called in the proper context, but now that we're 
+        // relying on native bind and/or an existing polyfill, we can't rely
+        // on the nuanced behavior of whatever `bind` implementation is on
+        // the page.
+        //
+        // MDC's polyfill, for instance, doesn't like binding methods that
+        // haven't got a `prototype` property defined.
+        value.valueOf = (function(method) {
+          return function() { return method.valueOf.call(method); };
+        })(method);
+
+        value.toString = (function(method) {
+          return function() { return method.toString.call(method); };
+        })(method);
       }
       this.prototype[property] = value;
     }