how to start web build

[yaoyiqun]

when i use this to build flutter for web in stable, here is the error.
jobs:
build:
name: Test on ${{ matrix.os }}
runs-on: ${{ matrix.os }}
strategy:
matrix:
os: [ubuntu-latest, windows-latest, macos-latest]
steps:
- uses: actions/checkout@v1
- uses: actions/setup-java@v1
with:
java-version: '12.x'
- uses: subosito/flutter-action@v1
with:
channel: 'stable'
- run: flutter pub get
- run: flutter test
- run: flutter config --enable-web
- run: flutter build web

Run flutter config --enable-web1s
You may need to restart any open editors for them to read new settings.
Run flutter config --enable-web
Setting "enable-web" value to "true".
You may need to restart any open editors for them to read new settings.

Run flutter build web2s
##[error]Process completed with exit code 1.
Run flutter build web
flutter build web
shell: /bin/bash -e {0}
env:
JAVA_HOME: /opt/hostedtoolcache/jdk/12.0.2/x64
JAVA_HOME_12.0.2_x64: /opt/hostedtoolcache/jdk/12.0.2/x64
FLUTTER_HOME: /opt/hostedtoolcache/flutter/1.12.13-hotfix.8-stable/x64
Downloading Web SDK... 0.5s
"build web" is not currently supported.
##[error]Process completed with exit code 1.

[yaoyiqun]

i know the reason , change channel to beta can solve it.