add GitHub workflow

.github/workflows/dart.yml

@@ -0,0 +1,22 @@
+name: Flutter CI
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build:
+    runs-on: 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: '1.9.1+hotfix.6'
+      - run: flutter pub get
+      - run: flutter test