Switch to "dart format" from "flutter format" (#922)

flutter format is deprecated

Author: brianquinlan

.github/workflows/cupertino.yml

@@ -36,7 +36,7 @@ jobs:
         name: Install dependencies
         run: flutter pub get
       - name: Check formatting
-        run: flutter format --output=none --set-exit-if-changed .
+        run: dart format --output=none --set-exit-if-changed .
         if: always() && steps.install.outcome == 'success'
       - name: Analyze code
         run: flutter analyze --fatal-infos

pkgs/cronet_http/tool/run_pigeon.sh

@@ -9,4 +9,4 @@ flutter pub run pigeon \
   --java_out android/src/main/java/io/flutter/plugins/cronet_http/Messages.java \
   --java_package "io.flutter.plugins.cronet_http"
 
-flutter format lib/src/messages.dart
+dart format lib/src/messages.dart