dart:html is deprecated.

Import the "web" package (https://pub.dev/packages/web) :

import "package:web/web.dart";

void navigate(String url, {String target = "_self"}) {
  if (target == "_blank") {
    window.open(url, "_blank")?.focus();
  } else {
    window.location.href = url;
  }
}

// In the current tab:
navigate("https://stackoverflow.com/questions/ask");
// In another tab:
navigate("https://stackoverflow.com/questions/ask", target: "_blank");