Not language features, but it lacks some HTML DOM objects. Eg. thead, tfoot constructors, style.setProperty don't work for css variables, etc. Small things, but

Not language features, but it lacks some HTML DOM objects. Eg. thead, tfoot constructors, style.setProperty don't work for css variables, etc. Small things, but extremely irritating because it defeats whole purpose of having sane wrapper around DOM API when it needs to be workarounded sometimes anyway. I recently tried to access window content opened by window.open and it was absolute nightmare. It cannot be done purely from Dart and requires JS interop, but what's worse passing browser objects between dart and js is buggy. I haven't found a solution that works both on dart2js and dartium, when it works on dartium it is broken on dart2js and vice versa. And debugging dart2js-generated code is almost impossible, just obfuscated mess.