fix dummy setters -- grammar requires an identifier

test/codegen/expect/async/async.js

 var async;
 (function(exports) {
   'use strict';
   dart.copyProperties(exports, {
     get _hasDocument() {
       return dart.equals(typeof document, 'object');
     }
   });
   // Function _invokeErrorHandler: (Function, Object, StackTrace) → dynamic
   function _invokeErrorHandler(errorHandler, error, stackTrace) {
@@ skipping 3746 matching lines @@
             sink._add(inputEvent);
             this._previous = inputEvent;
           }
         }
       }
     }
     dart.defineLazyProperties(_DistinctStream, {
       get _SENTINEL() {
         return new core.Object();
       },
-      set _SENTINEL() {}
+      set _SENTINEL(_) {}
     });
     return _DistinctStream;
   });
   let _DistinctStream = _DistinctStream$(dynamic);
   let _EventSinkWrapper$ = dart.generic(function(T) {
     class _EventSinkWrapper extends dart.Object {
       _EventSinkWrapper(_sink) {
         this._sink = _sink;
       }
       add(data) {
@@ skipping 930 matching lines @@
     }
     print(line) {
       _internal.printToConsole(line);
     }
   }
   _RootZone._rootDelegate = null;
   dart.defineLazyProperties(_RootZone, {
     get _rootMap() {
       return new collection.HashMap();
     },
-    set _rootMap() {}
+    set _rootMap(_) {}
   });
   let _ROOT_ZONE = new _RootZone();
   // Function runZoned: (() → dynamic, {zoneValues: Map<dynamic, dynamic>, zoneSpecification: ZoneSpecification, onError: Function}) → dynamic
   function runZoned(body, opt$) {
     let zoneValues = opt$.zoneValues === void 0 ? null : opt$.zoneValues;
     let zoneSpecification = opt$.zoneSpecification === void 0 ? null : opt$.zoneSpecification;
     let onError = opt$.onError === void 0 ? null : opt$.onError;
     let errorHandler = null;
     if (onError !== null) {
       errorHandler = (self, parent, zone, error, stackTrace) => {
@@ skipping 52 matching lines @@
   exports.StreamIterator$ = StreamIterator$;
   exports.StreamController = StreamController;
   exports.StreamController$ = StreamController$;
   exports.Timer = Timer;
   exports.AsyncError = AsyncError;
   exports.ZoneSpecification = ZoneSpecification;
   exports.ZoneDelegate = ZoneDelegate;
   exports.Zone = Zone;
   exports.runZoned = runZoned;
 })(async || (async = {}));