You can use Flutter as an imperative framework. Just ignore widgets (the reactive top layer of Flutter) and use the underlying imperative elements directly.
Well, I'd say the main value proposition of Qt/Delphi/wxWidgets/WinForms/etc... is a rich set of widgets/controls including its event handling and layout mechanisms. Is such stuff available in the imperative impl of Flutter?
That's Flutter, the framework. I like Dart and I wish some conventional imperative Qt/Delphi-like GUI toolkit was available for it.