build: 🔨 Adding flutterw for consitent flutter version

.gitmodules

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+[submodule ".flutter"]
+	path = .flutter
+	url = https://github.com/flutter/flutter.git
+	branch = stable

flutterw

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+#!/usr/bin/env sh
+
+##############################################################################
+##
+##  Flutter start up script for UN*X
+##  Version: v1.1.2
+##  Date: 2021-03-09 13:41:00
+##
+##  Use this flutter wrapper to bundle Flutter within your project to make
+##  sure everybody builds with the same version.
+##
+##  Read about the install and uninstall process in the README on GitHub
+##  https://github.com/passsy/flutter_wrapper
+##
+##  Inspired by gradle-wrapper.
+##
+##############################################################################
+
+# Attempt to set APP_HOME
+# Resolve links: $0 may be a link
+PRG="$0"
+# Need this for relative symlinks.
+while [ -h "$PRG" ]; do
+  ls=$(ls -ld "$PRG")
+  link=$(expr "$ls" : '.*-> \(.*\)$')
+  if expr "$link" : '/.*' >/dev/null; then
+    PRG="$link"
+  else
+    PRG=$(dirname "$PRG")"/$link"
+  fi
+done
+SAVED="$(pwd)"
+cd "$(dirname "$PRG")/" >/dev/null
+APP_HOME="$(pwd -P)"
+cd "$SAVED" >/dev/null
+
+FLUTTER_SUBMODULE_NAME='.flutter'
+FLUTTER_DIR="$APP_HOME/$FLUTTER_SUBMODULE_NAME"
+
+# by default we should be in the correct project dir, but when run from Finder on Mac, the cwd is wrong
+if [ "$(uname)" = "Darwin" ] && [ "$HOME" = "$PWD" ]; then
+  cd "$(dirname "$0")"
+fi
+
+# submodule starting with "-" are not initialized
+init_status=$(git submodule | grep "\ $FLUTTER_SUBMODULE_NAME$" | cut -c 1)
+
+# Fix not initialized flutter submodule
+if [ "$init_status" = "-" ]; then
+  echo "$FLUTTER_SUBMODULE_NAME submodule not initialized. Initializing..."
+  git submodule update --init ${FLUTTER_SUBMODULE_NAME}
+fi
+
+# Detect detach HEAD and fix it. commands like upgrade expect a valid branch, not a detached HEAD
+FLUTTER_SYMBOLIC_REF=$(git -C ${FLUTTER_SUBMODULE_NAME} symbolic-ref -q HEAD)
+if [ -z ${FLUTTER_SYMBOLIC_REF} ]; then
+  FLUTTER_REV=$(git -C ${FLUTTER_SUBMODULE_NAME} rev-parse HEAD)
+  FLUTTER_CHANNEL=$(git config -f .gitmodules submodule.${FLUTTER_SUBMODULE_NAME}.branch)
+
+  if [ -z $FLUTTER_CHANNEL ]; then
+    echo "Warning: Submodule '$FLUTTER_SUBMODULE_NAME' doesn't point to an official Flutter channel \
+            (one of stable|beta|dev|master). './flutterw upgrade' will fail without a channel."
+    echo "Fix this by adding a specific channel with:"
+    echo "\t- './flutterw channel <channel>' or"
+    echo "\t- Add 'branch = <channel>' to '$FLUTTER_SUBMODULE_NAME' submodule in .gitmodules"
+  else
+    echo "Fixing detached HEAD: '$FLUTTER_SUBMODULE_NAME' submodule points to a specific commit $FLUTTER_REV, not channel '$FLUTTER_CHANNEL' (as defined in .gitmodules)."
+    # Make sure channel is fetched
+    # Remove old channel branch because it might be moved to an unrelated commit where fast-forward pull isn't possible
+    git -C ${FLUTTER_SUBMODULE_NAME} branch -q -D ${FLUTTER_CHANNEL} 2> /dev/null || true
+    git -C ${FLUTTER_SUBMODULE_NAME} fetch -q origin
+
+    # bind current HEAD to channel defined in .gitmodules
+    git -C ${FLUTTER_SUBMODULE_NAME} checkout -q -b ${FLUTTER_CHANNEL} ${FLUTTER_REV}
+    git -C ${FLUTTER_SUBMODULE_NAME} branch -q -u origin/${FLUTTER_CHANNEL} ${FLUTTER_CHANNEL}
+    echo "Fixed! Migrated to channel '$FLUTTER_CHANNEL' while staying at commit $FLUTTER_REV. './flutterw upgrade' now works without problems!"
+    git -C ${FLUTTER_SUBMODULE_NAME} status -bs
+  fi
+fi
+
+# Wrapper tasks done, call flutter binay with all args
+set -e
+"$FLUTTER_DIR/bin/flutter" "$@"
+
+# Post flutterw tasks. exit code from /bin/flutterw will be used as final exit
+FLUTTER_EXIT_STATUS=$?
+if [ ${FLUTTER_EXIT_STATUS} -eq 0 ]; then
+
+  # ./flutterw channel CHANNEL
+  if echo "$@" | grep -q "channel"; then
+    # make sure .gitmodules is updated as well
+    CHANNEL=${2} # second arg
+    git config -f .gitmodules submodule.${FLUTTER_SUBMODULE_NAME}.branch ${CHANNEL}
+    # makes sure nobody forgets to do commit all changed files
+    git add .gitmodules
+    git add ${FLUTTER_SUBMODULE_NAME}
+  fi
+
+  # ./flutterw upgrade
+  if echo "$@" | grep -q "upgrade"; then
+    # makes sure nobody forgets to do commit the changed submodule
+    git add ${FLUTTER_SUBMODULE_NAME}
+    # flutter packages get runs automatically. Stage those changes as well
+    git add pubspec.lock
+  fi
+fi
+
+exit ${FLUTTER_EXIT_STATUS}