Flutter format is removed, now dart format (#168)

It was removed in 3.12: https://github.com/flutter/flutter/pull/129360

This updates the README and our CI job.

.github/workflows/flutterfmt.sh

@@ -1,10 +0,0 @@
-#!/bin/sh
-DIRS="lib test"
-EXIT=0
-
-for DIR in $DIRS; do
-    OUT="$(flutter format -l 120 --suppress-analytics "$DIR" | sed -e "s/^Formatted \(.*\)/::error file=$DIR\/\1::Not formatted/g")"
-    echo "$OUT" | grep "::error" && EXIT=1
-done
-
-exit $EXIT

.github/workflows/flutterfmt.yml

@@ -23,5 +23,5 @@ jobs:
       - name: Check out code
         uses: actions/checkout@v3
 
-      - name: flutter format
+      - name: Check formating
-        run: $GITHUB_WORKSPACE/.github/workflows/flutterfmt.sh
+        run: dart format -l120 lib/ --set-exit-if-changed --suppress-analytics --output none

README.md

@@ -32,13 +32,16 @@ If you are having issues with iOS pods, try blowing it all away! `cd ios && rm -
 
 # Formatting
 
-`flutter format` can be used to format the code in `lib` and `test` but it's default is 80 char line limit, it's 2020
+`dart format` can be used to format the code in `lib` and `test`.  We use a line-length of 120 characters.
 
 Use:
 ```sh
-flutter format lib/ test/ -l 120
+dart format lib/ test/ -l 120
 ```
 
+In Android Studio, set the line length using Preferences -> Editor -> Code Style -> Dart -> Line length, set it to 120.  Enable auto-format with Preferences -> Languages & Frameworks -> Flutter -> Format code on save.
+
+
 # Release
 
 Update `version` in `pubspec.yaml` to reflect this release, then