implement mixins in subtype checks, more codegen fixes

lib/runtime/dart/_isolate_helper.js

 var _isolate_helper;
 (function(exports) {
   'use strict';
   // Function _serializeMessage: (dynamic) → dynamic
   function _serializeMessage(message) {
     return new _Serializer().serialize(message);
   }
   // Function _deserializeMessage: (dynamic) → dynamic
   function _deserializeMessage(message) {
     return new _Deserializer().deserialize(message);
@@ skipping 808 matching lines @@
     },
     get globalWorker() {
       _js_helper.requiresPreamble();
       return self.Worker;
     },
     get globalPostMessageDefined() {
       _js_helper.requiresPreamble();
       return !!self.postMessage;
     }
   });
+  class _MainFunction extends core.Function {}
+  class _MainFunctionArgs extends core.Function {}
+  class _MainFunctionArgsMessage extends core.Function {}
   let _getEventData = Symbol('_getEventData');
   let _getJSFunctionFromName = Symbol('_getJSFunctionFromName');
   let _startIsolate = Symbol('_startIsolate');
   let _log = Symbol('_log');
   let _consoleLog = Symbol('_consoleLog');
   let _getJSFunctionName = Symbol('_getJSFunctionName');
   let _allocate = Symbol('_allocate');
   let _startWorker = Symbol('_startWorker');
   let _startNonWorker = Symbol('_startNonWorker');
   let _spawnWorker = Symbol('_spawnWorker');
@@ skipping 533 matching lines @@
   exports.leaveJsAsync = leaveJsAsync;
   exports.isWorker = isWorker;
   exports.startRootIsolate = startRootIsolate;
   exports.IsolateNatives = IsolateNatives;
   exports.RawReceivePortImpl = RawReceivePortImpl;
   exports.ReceivePortImpl = ReceivePortImpl;
   exports.TimerImpl = TimerImpl;
   exports.hasTimer = hasTimer;
   exports.CapabilityImpl = CapabilityImpl;
 })(_isolate_helper || (_isolate_helper = {}));