Flutter dart debugger breakpoints stopped working

I'm learning Dart and Flutter and developing a small Android Flutter app under Android Studio 3.1.2. Suddenly debugger breakpoints stopped working - the app started in debug mode never stops on them, and the red dots indicating breakpoints change to red dots with x inside. The only place in my app where they still work is in the main.dart module.

I cleaned the project many times, tested on two different devices, uninstalled completely my Flutter debug app and started new - nothing helps. Flutter update on beta (current beta 2) channel does nothing. Also tried switching to dev and master channels - no help.

Did anyone encounter a similar situation, how do deal with it?

Edit, adding mail.dart and output of flutter doctor (currently on master branch, but have same problem after switch to beta or dev branches):

Imports from main.dart, or better all of main.dart:

import 'package:flutter/material.dart';
import 'package:flutter_localizations/flutter_localizations.dart';

import 'app_strings.dart';
import 'net_libs_page/net_libs_page.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
// This widget is the root of your application.
@override
Widget build(BuildContext context) {
return MaterialApp(
localizationsDelegates: [
// ... app-specific localization delegate[s] here
AppLocalizationsDelegate(),
GlobalMaterialLocalizations.delegate,
GlobalWidgetsLocalizations.delegate,
//FallbackMaterialLocalisationsDelegate(),
],
supportedLocales: [
Locale('en', 'US'), // English
Locale('es', 'ES'), // Spanish
Locale('pl', 'PL'),
// ... other locales the app supports
],
title: '@Voice Network Library', //_strings.title,
theme: ThemeData(
// This is the theme of your application.
//
// Try running your application with "flutter run". You'll see the
// application has a blue toolbar. Then, without quitting the app, try
// changing the primarySwatch below to Colors.green and then invoke
// "hot reload" (press "r" in the console where you ran "flutter run",
// or press Run > Flutter Hot Reload in IntelliJ). Notice that the
// counter didn't reset back to zero; the application is not restarted.
primarySwatch: Colors.indigo,
),
home: NetLibsPage(),
);
}
}


Flutter doctor output:

Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel master, v0.4.5-pre.52, on Microsoft Windows [Version 10.0.16299.431], locale en-US)
[√] Android toolchain - develop for Android devices (Android SDK 27.0.3)
[√] Android Studio (version 3.1)
[!] IntelliJ IDEA Ultimate Edition (version 2018.1)
X Flutter plugin not installed; this adds Flutter specific functionality.
X Dart plugin not installed; this adds Dart specific functionality.
[!] VS Code, 32-bit edition (version 1.19.3)
[√] Connected devices (1 available)

! Doctor found issues in 2 categories.


Don't use relative imports in lib/main.dart

import 'app_strings.dart';
import 'net_libs_page/net_libs_page.dart';


instead use

import 'package:my_app/app_strings.dart';
import 'package:my_app/net_libs_page/net_libs_page.dart';


where my_app is what is in pubspec.yaml in the name: field.

The issue where this is tracked https://github.com/dart-lang/sdk/issues/33076