Suggest imports from custom directories

[hyungtaecf]

Issue Description

I'm using auto-generated .arb and .dart files for localization in Flutter. It is generated under the directory .dart_tool/flutter_gen/

When using Quick Fix to find AppLocalizations.of(context) it doesn't suggest the import that should be like following:
import 'package:flutter_gen/gen_l10n/app_localizations.dart';

This import is needed for basically every file in the UI that displays text, so it would help a lot if it was possible to get suggested imports from custom directories outside lib/ as well.

What is the fix

Please make a way to customize the directory scope of the import suggestions or make it automatically somehow.

Other information

For now, I am handling this by making another file within the project with the following code:
AppLocalizations l10nOf(BuildContext context) => AppLocalizations.of(context);

[scheglov]

What is the mechanism that adds flutter_gen to a package?
Is .dart_tool/package_config.json somehow updated?
Is there any signal in pubspec.yaml that will say that this package requires such package:flutter_gen generation?

[asashour]

As specified in Flutter internationalization, there are two things in pubspec.yaml:

dependencies:
  flutter_localizations:
    sdk: flutter    # this

flutter:
  generate: true    # and this

[DanTup]

There's an open issue at flutter/flutter#102983 about re-thinking how flutter_gen works because it causes quite a few problems (for ex. this also came up at Dart-Code/Dart-Code#3948).