When running dart2js on the following program

main() {
  var o = new Object();
  print((1 + o.hashCode) & 0xFF);
}

I get the warning

test.dart:3:9: Warning: No operator '&' in class 'num'.
  print((1 + o.hashCode) & 0xFF);

If o is types as Object the warning goes away. That is compiling the following two programs gives no warnings.

main() {
  Object o = new Object();
  print((1 + o.hashCode) & 0xFF);
}

main() {
  print((1 + new Object().hashCode) & 0xFF);
}