fix dummy setters -- grammar requires an identifier

test/codegen/expect/_isolate_helper/_isolate_helper.js

 var _isolate_helper;
 (function(exports) {
   'use strict';
   // Function _callInIsolate: (_IsolateContext, Function) → dynamic
   function _callInIsolate(isolate, function) {
     let result = isolate.eval(function);
     exports._globalState.topEventLoop.run();
     return result;
   }
   // Function enterJsAsync: () → dynamic
@@ skipping 690 matching lines @@
       }
       onError(message);
       return true;
     }
   }
   IsolateNatives.enableSpawnWorker = null;
   dart.defineLazyProperties(IsolateNatives, {
     get thisScript() {
       return computeThisScript();
     },
-    set thisScript() {},
+    set thisScript(_) {},
     get workerIds() {
       return new core.Expando();
     }
   });
   class _BaseSendPort extends dart.Object {
     _BaseSendPort(_isolateId) {
       this._isolateId = _isolateId;
     }
     _checkReplyTo(replyTo) {
       if (dart.notNull(dart.notNull(replyTo !== null) && dart.notNull(!dart.is(replyTo, _NativeJsSendPort))) && dart.notNull(!dart.is(replyTo, _WorkerSendPort))) {
@@ skipping 544 matching lines @@
   exports.globalWindow = globalWindow;
   exports.globalWorker = globalWorker;
   exports.globalPostMessageDefined = globalPostMessageDefined;
   exports.IsolateNatives = IsolateNatives;
   exports.RawReceivePortImpl = RawReceivePortImpl;
   exports.ReceivePortImpl = ReceivePortImpl;
   exports.TimerImpl = TimerImpl;
   exports.hasTimer = hasTimer;
   exports.CapabilityImpl = CapabilityImpl;
 })(_isolate_helper || (_isolate_helper = {}));