fixes #52, fields shadowing getters/setters or other fields

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 1178 matching lines @@
       }
     }
     ['=='](other) {
       return dart.is(other, _WorkerSendPort) && dart.equals(this[_workerId], dart.dload(other, _workerId)) && dart.equals(this[_isolateId], dart.dload(other, _isolateId)) && dart.equals(this[_receivePortId], dart.dload(other, _receivePortId));
     }
     get hashCode() {
       return dart.notNull(this[_workerId]) << 16 ^ dart.notNull(this[_isolateId]) << 8 ^ dart.notNull(this[_receivePortId]);
     }
   }
   _WorkerSendPort[dart.implements] = () => [isolate.SendPort];
+  let _nextFreeId = Symbol('_nextFreeId');
   let _handler = Symbol('_handler');
-  let _nextFreeId = Symbol('_nextFreeId');
   class RawReceivePortImpl extends core.Object {
     RawReceivePortImpl(handler) {
       this[_handler] = handler;
       this[_id] = (() => {
         let x = RawReceivePortImpl[_nextFreeId];
         RawReceivePortImpl[_nextFreeId] = dart.notNull(x) + 1;
         return x;
       })();
       this[_isClosed] = false;
       exports._globalState.currentContext.register(this[_id], this);
@@ skipping 181 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 = {}));