Doctor

[√] Flutter (Channel master, v1.13.1-pre.59, on Microsoft Windows [versão 10.0.17763.864], locale pt-BR)
• Flutter version 1.13.1-pre.59 at C:\flutter
• Framework revision e58dc16 (8 hours ago), 2019-12-06 18:21:52 -0800
• Engine revision e7b69ce
• Dart version 2.7.0 (build 2.7.0-dev.2.1 a9c77229c2)

[√] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
• Android SDK at C:\Users\usuario\AppData\Local\Android\sdk
• Android NDK location not configured (optional; useful for native profiling support)
• Platform android-29, build-tools 29.0.2
• Java binary at: C:\android-studio\jre\bin\java
• Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b04)
• All Android licenses accepted.

[√] Chrome - develop for the web
• Chrome at C:\Program Files (x86)\Google\Chrome\Application\chrome.exe

[!] Android Studio (version 4.0)
• Android Studio at C:\android-studio
X Flutter plugin not installed; this adds Flutter specific functionality.
X Dart plugin not installed; this adds Dart specific functionality.
• Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b04)

[√] VS Code (version 1.40.2)
• VS Code at C:\Users\usuario\AppData\Local\Programs\Microsoft VS Code
• Flutter extension version 3.7.0

[√] Connected device (2 available)
• Chrome • chrome • web-javascript • Google Chrome 78.0.3904.108
• Web Server • web-server • web-javascript • Flutter Tools

! Doctor found issues in 1 category.

Code:

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(home: MyHomePage());
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.green,
      appBar: AppBar(),
      body: Padding(
        padding: const EdgeInsets.all(30),
        child: ListView.separated(
          itemCount: 30,
          itemBuilder: (BuildContext context, int index) {
            return ListTile(
              title: Text('aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'),
            );
          },
          separatorBuilder: (BuildContext context, int index) => const Divider(
            color: Colors.red,
          ),
        ),
      ),
    );
  }
}

Issue:

Divider not render in some cases, on the exemple code divider not render.
on canvas back end (--dart-define=FLUTTER_WEB_USE_SKIA=true) works fine