New Flutter Project wizard not showing on Android Studio 3.0.1

If you are sure to have both Dart and Flutter plugins correctly installed, check again your Plugins and be sure that Android APK Support is enabled.
If it isn't, enable it!

Et voilà! Now everything should be fine:

You can find the plugin menu inside the Welcome page

or, when you have a project opened, inside the Preferences menu:

Had the same problem. Check-in Android Studio if the Dart and Flutter plugins are both installed and marked with a lock symbol in Preferences --> Plugins.

Anyway, the following procedure helped me:

1. uninstall the Flutter plugin
2. restart Android Studio.
3. uninstall the Dart plugin
4. restart Android Studio again which seemed important to do
5. install the Dart plugin again
6. restart Android Studio although it is annoying
7. install the Flutter plugin again
8. and guess: restart Android Studio

After the last restart I saw the success message:

I assume we both installed both plugins without restarting after Dart.

You need check the “AndroidAPK Support” in your Plugins.

See this screenshot: