fix(zone): fix conflict with Polymer elements

I noticed when including Polymer 1.0 elements in an Angular 2 app, this line throws an error saying it can't find `value` of undefined. I've added a simple workaround which seems to fix the issue and (so far at least) doesn't seem to cause any additional issues with Polymer or Angular.

cc @justinfagnani @addyosmani

lib/patch/register-element.js

@@ -21,7 +21,7 @@ function apply() {
       callbacks.forEach(function (callback) {
         if (opts.prototype.hasOwnProperty(callback)) {
           var descriptor = Object.getOwnPropertyDescriptor(opts.prototype, callback);
-          if (descriptor.value) {
+          if (descriptor && descriptor.value) {
             descriptor.value = global.zone.bind(descriptor.value);
             _redefineProperty(opts.prototype, callback, descriptor);
           } else {