Add support for .dart files

do-reformat.sh

@@ -33,6 +33,10 @@ elif [[ $file_type == "golang" ]] ; then
     out=$(gofmt -w "$file" 2>/dev/null)
     retval="$?"
     success_retval=0
+elif [[ $file_type == "dart" ]] ; then
+    out=$(dart format $file)
+    retval="$?"
+    success_retval=0
 else
     echo -n "ignoring"
     exit 0