This is a complete Workflow file for Flutter apps. It supports build and release for all platforms.

Flutter Workflow.yaml

name: Flutter Test Analysis Build

on:
  push:
    branches: [master]
  pull_request:
    branches: [master]

jobs:
  flutter_test:
    name: Flutter Test and Analysis
    runs-on: ubuntu-latest

    steps:
      - name: Checkout
        uses: actions/checkout@v2

      - name: Setup Java
        uses: actions/setup-java@v2
        with:
          distribution: 'zulu'
          java-version: '11'

      - name: Setup Flutter
        uses: subosito/flutter-action@v2
        with:
          channel: "stable"

      - name: Version
        run: flutter --version

      - name: Doctor
        run: flutter doctor

      - name: Pub Get
        run: flutter pub get

      - name: Analyze
        run: flutter analyze

      - name: Test
        run: flutter test

  build_ios:
    name: Build Flutter (iOS)
    needs: [flutter_test]
    runs-on: macos-latest

    steps:
      - name: Checkout
        uses: actions/checkout@v2

      - name: Setup Flutter
        uses: subosito/flutter-action@v2
        with:
          channel: "stable"

      - name: Build iOS
        run: |
          flutter build ios --no-codesign
          cd build/ios/iphoneos
          mkdir Payload
          cd Payload
          ln -s ../Runner.app
          cd ..
          zip -r app.ipa Payload

      - name: Publish Artifacts
        uses: actions/upload-artifact@v2
        with:
          name: assets-for-download
          path: build/ios/iphoneos/app.ipa

  build_android:
    name: Build Flutter (Android)
    needs: [flutter_test]
    runs-on: ubuntu-latest

    steps:
      - name: Checkout
        uses: actions/checkout@v2

      - name: Setup Java
        uses: actions/setup-java@v2
        with:
          distribution: 'zulu'
          java-version: '11'

      - name: Setup Flutter
        uses: subosito/flutter-action@v2
        with:
          channel: "stable"

      - name: Build App Bundle
        run: flutter build appbundle

      - name: Build Apk
        run: flutter build apk --release

      - name: Publish Artifacts
        uses: actions/upload-artifact@v2
        with:
          name: assets-for-download
          path: |
            build/app/outputs/bundle/release/app-release.aab
            build/app/outputs/flutter-apk/app-release.apk

  build_web:
    name: Build Flutter (Web)
    needs: [flutter_test]
    runs-on: ubuntu-latest

    steps:
      - name: Checkout
        uses: actions/checkout@v2

      - name: Setup Flutter
        uses: subosito/flutter-action@v2
        with:
          channel: "stable"

      - name: Build web
        run: flutter build web --release

      - name: Zip
        run: |
          cd build
          zip -r web.zip web

      - name: Upload web artifact 
        uses: actions/upload-artifact@v1
        with:
          name: assets-for-download
          path: build/web.zip

  build_windows:
    name: Build Flutter (Windows)
    needs: [flutter_test]
    runs-on: windows-latest

    steps:
      - name: Checkout
        uses: actions/checkout@v2

      - name: Setup Flutter
        uses: subosito/flutter-action@v2
        with:
          channel: "stable"

      - name: Config Windows
        run: flutter config --enable-windows-desktop

      - name: Build Windows
        run: flutter build windows

      - name: Zip
        run: |
            cd build/windows/runner/Release
            7z a -r ../Release *
            cd..

      - name: Publish Artifacts
        uses: actions/upload-artifact@v2
        with:
          name: assets-for-download
          path: build/windows/runner/Release.7z

  build_linux:
    name: Build Flutter (Linux)
    needs: [flutter_test]
    runs-on: ubuntu-latest

    steps:
      - name: Checkout
        uses: actions/checkout@v2

      - name: Setup Flutter
        uses: subosito/flutter-action@v2
        with:
          channel: "stable"

      - name: Install Dependencies
        run: |
          sudo apt-get update -y
          sudo apt-get install -y ninja-build libgtk-3-dev

      - name: Config Linux
        run: flutter config --enable-linux-desktop

      - name: Build Linux
        run: flutter build linux --release

      - name: Zip
        run: |
          cd build/linux/x64/release
          zip -r linux.zip bundle

      - name: Upload web artifact 
        uses: actions/upload-artifact@v1
        with:
          name: assets-for-download
          path: build/linux/x64/release/linux.zip

  build_macos:
    name: Build Flutter (Macos)
    needs: [flutter_test]
    runs-on: macos-latest

    steps:
      - name: Checkout
        uses: actions/checkout@v2

      - name: Setup Flutter
        uses: subosito/flutter-action@v2
        with:
          channel: "stable"

      - name: Config Macos
        run: flutter config --enable-macos-desktop

      - name: Build Macos
        run: flutter build macos --release

      - name: Zip
        run: |
          cd build/macos/Build/Products
          zip -r macos.zip Release

      - name: Publish Artifacts
        uses: actions/upload-artifact@v2
        with:
          name: assets-for-download
          path: build/macos/Build/Products/macos.zip

  publish_release:
    name: Publish Build files to releases
    needs: [flutter_test, build_macos, build_linux, build_windows, build_web, build_android, build_ios]
    runs-on: ubuntu-latest

    steps:
      - name: Downlaod Artifacts
        uses: actions/download-artifact@v2
        with:
          name: assets-for-download

      - name: Push Files to Releases
        uses: ncipollo/release-action@v1
        with:
            artifacts: "app.ipa, flutter-apk/app-release.apk, bundle/release/app-release.aab, linux.zip, macos.zip, Release.7z, web.zip"
            tag: v1.${{ github.run_number }}
            token: ${{ secrets.TOKEN }}