Update dart.yml

Author: shiburagi

.github/workflows/dart.yml

@@ -12,15 +12,15 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest, windows-latest, macos-latest]
+        os: [ubuntu-latest]
     steps:
       - uses: actions/checkout@v1
       - uses: actions/setup-java@v1
         with:
           java-version: '12.x'
       - uses: subosito/flutter-action@v1
         with:
-          flutter-version: '1.11.0'
+#           flutter-version: '1.11.0'
           channel: 'beta'
       - run: dart --version
       - run: flutter --version