Emit forwarding getter/setter when overriding just a getter or setter.

test/codegen/expect/extensions.js

 dart_library.library('extensions', null, /* Imports */[
   'dart_sdk'
 ], function(exports, dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const collection = dart_sdk.collection;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const extensions = Object.create(null);
   extensions.StringIterable = class StringIterable extends collection.IterableBase$(core.String) {
     StringIterable() {
       this.iterator = null;
       super.IterableBase();
     }
   };
-  dart.virtualField(extensions.StringIterable, 'iterator');
   dart.setSignature(extensions.StringIterable, {});
   dart.defineExtensionMembers(extensions.StringIterable, ['iterator']);
   extensions.main = function() {
     return new extensions.StringIterable();
   };
   dart.fn(extensions.main);
   // Exports:
   exports.extensions = extensions;
 });