flutter format -> dart format (#846)

tool/flutter_ci_script_shared.sh

@@ -1,14 +1,6 @@
 function ci_projects () {
     local channel="$1"
 
-    # The `-n` option has been replaced with a more flexible `-o` for "output"
-    # in dev.
-    if [ $channel == "dev" ]; then
-        format_option="-o none"
-    else
-        format_option="-n"
-    fi
-
     shift
     local arr=("$@")
     for PROJECT_NAME in "${arr[@]}"
@@ -23,7 +15,7 @@ function ci_projects () {
         flutter analyze
 
         # Run the formatter on all the dart files to make sure everything's linted.
-        flutter format $format_option --set-exit-if-changed .
+        dart format --output none --set-exit-if-changed .
 
         # Run the actual tests.
         if [ -d "test" ]