Fix increment on nullable

test/codegen/expect/dart/isolate.js

 var isolate;
 (function(exports) {
   'use strict';
   class IsolateSpawnException extends dart.Object {
     IsolateSpawnException(message) {
       this.message = message;
     }
     toString() {
       return `IsolateSpawnException: ${this.message}`;
     }
@@ skipping 20 matching lines @@
       }
 
     }
     static spawnUri(uri, args, message, opt$) {
       let paused = opt$.paused === void 0 ? false : opt$.paused;
       let packageRoot = opt$.packageRoot === void 0 ? null : opt$.packageRoot;
       if (packageRoot !== null)
         throw new core.UnimplementedError("packageRoot");
       try {
         if (dart.is(args, core.List$(core.String))) {
-          for (let i = 0; dart.notNull(i) < dart.notNull(args.length); dart.notNull(i)++) {
+          for (let i = 0; dart.notNull(i) < dart.notNull(args.length); i = dart.notNull(i) + 1) {
             if (!(typeof args.get(i) == string)) {
               throw new core.ArgumentError(`Args must be a list of Strings ${args}`);
             }
           }
         } else if (args !== null) {
           throw new core.ArgumentError(`Args must be a list of Strings ${args}`);
         }
         return dart.as(_isolate_helper.IsolateNatives.spawnUri(uri, args, message, paused).then((msg) => new Isolate(dart.as(dart.dindex(msg, 1), SendPort), {pauseCapability: dart.as(dart.dindex(msg, 2), Capability), terminateCapability: dart.as(dart.dindex(msg, 3), Capability)})), async.Future$(Isolate));
       } catch (e) {
         let st = dart.stackTrace(e);
@@ skipping 150 matching lines @@
   }
   // Exports:
   exports.IsolateSpawnException = IsolateSpawnException;
   exports.Isolate = Isolate;
   exports.SendPort = SendPort;
   exports.ReceivePort = ReceivePort;
   exports.RawReceivePort = RawReceivePort;
   exports.RemoteError = RemoteError;
   exports.Capability = Capability;
 })(isolate || (isolate = {}));