Describe the bug
Snackbar not working as expected .
Whenever snackbar shows up , Its shows 2 snackbar both of different looks and size,
one uses whole app and other uses only demo section .

To switch channel run:

flutter channel master
flutter upgrade

Flutter doctor
Copy and paste the output of flutter doctor -v here

Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel master, 1.24.0-8.0.pre.266, on Microsoft Windows [Version 10.0.19042.630], locale en-US)
[√] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
[√] Visual Studio - develop for Windows (Visual Studio Community 2019 16.8.1)
[√] Android Studio (version 4.1.0)
[√] VS Code (version 1.51.1)
[√] Connected device (1 available)

• No issues found!

To Reproduce
Steps to reproduce the behavior:

1. Go to 'Material demo section'
2. Click on 'snackbar demo'
3. Click on 'SHOW A SNACKBAR' button
4. See error

Expected behavior
Only one snackbar should show up and it should only use demo area .

Screenshots

Desktop (please complete the following information):

• OS: Windows, Android

Smartphone (please complete the following information):

• Device: Oneplus 7 pro
• OS: Android 10

Additional context
Issue caused after merging this PR #343 .