The program:

import 'dart:async';

p() async* {
  yield [1, 2, 3];
}

Stream<int> foo(Stream stream) async* {
  await for (List data in stream) {
    print('Will iterate ${data} ${data.runtimeType}');
    for (var i in data) {
      print('got $i');
    }
  }
}

main() async {
  var c;
  c = new StreamController(onListen: () async {
    c.add([1,2,3]);
  });
// foo(p()).listen((v) {});
  foo(c.stream).listen((v) {});
}

On the VM prints:
Will iterate [1, 2, 3] List
got 1
got 2
got 3

But on dart2js it prints:
Will iterate [1, 2, 3] JSArray

The js for the inner for loop (before async-translation) is

      for (t2 = J.get$iterator$a(data); t2.moveNext$0(), false;) {
        i = t2.get$current();
        line = "got " + H.S(i);
        H.printString(line);
      }

I am not sure where the , false comes from. It seems to be an inferrence problem. If the commented line is inserted the problem disappears.