You said it yourself - 'Dart editor'. It's not an IDE and unlike .NET it has no supplied UI toolkits like WinForms or WPF or whatever. Dart itself is a general purpose language. Presumably there are third-party, people working on various UI toolkits and designers, or bindings for existing ones like QT. But I don't think you'll see one from Google anytime soon.
This is essentially the same situation as the early days of Java, and similar indeed to .NET - the difference is that there are official GUI toolkits from Microsoft.