implement rethrow

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 236 matching lines @@
       let result = null;
       this._isExecutingEvent = true;
       try {
         result = dart.dinvokef(code);
       } catch (e) {
         let s = dart.stackTrace(e);
         this.handleUncaughtError(e, s);
         if (this.errorsAreFatal) {
           this.kill();
           if (core.identical(this, exports._globalState.rootContext)) {
-            dart.throw_("Unimplemented RethrowExpression: rethrow");
+            throw e;
           }
         }
       }
  finally {
         this._isExecutingEvent = false;
         exports._globalState.currentContext = old;
         if (old !== null)
           old._setGlobals();
         if (this._scheduledControlEvents !== null) {
           while (dart.dload(this._scheduledControlEvents, 'isNotEmpty')) {
@@ skipping 998 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 = {}));