only export fetch, Headers, Request, Response for browser

To ensure parity with Fastboot polyfills, supported properties should only be used internally in github/fetch but not exported.

assets/browser-fetch.js.t

@@ -9,7 +9,6 @@
       'Blob',
       'URLSearchParams',
       'Symbol',
-      'iterator',
       'ArrayBuffer'
     ];
     var polyfillProps = [
@@ -54,6 +53,10 @@
     } else {
       self['default'] = self.fetch;
     }
+
+    supportProps.forEach(function(prop) {
+      delete self[prop];
+    });
   });
 
   define('fetch/ajax', ['exports'], function() {