add a dev mode to the flutter script

bin/flutter

@@ -15,6 +15,11 @@ SCRIPT_PATH="$FLUTTER_TOOLS_DIR/bin/flutter_tools.dart"
 # TODO(abarth): We shouldn't require dart to be on the user's path.
 DART=dart
 
+if [ -n $FLUTTER_DEV ]; then
+  $DART --checked --package-root="$FLUTTER_TOOLS_DIR/packages" "$SCRIPT_PATH" "$@"
+  exit 0
+fi
+
 REVISION=`(cd "$FLUTTER_ROOT"; git rev-parse HEAD)`
 if [ ! -f "$SNAPSHOT_PATH" ] || [ ! -f "$STAMP_PATH" ] || [ `cat "$STAMP_PATH"` != "$REVISION" ] || [ "$FLUTTER_TOOLS_DIR/pubspec.yaml" -nt "$FLUTTER_TOOLS_DIR/pubspec.lock" ]; then
   echo Updating flutter tool...

bin/flutter.bat

@@ -13,6 +13,11 @@ SET script_path=%flutter_tools_dir%\bin\flutter_tools.dart
 REM TODO: Don't require dart to be on the user's path
 SET dart=dart
 
+IF "%FLUTTER_DEV%" == "" GOTO no_flutter_dev
+CALL %dart% --checked --package-root="%flutter_tools_dir%\packages" "%script_path%" %*
+EXIT /B %ERRORLEVEL%
+:no_flutter_dev
+
 REM Set current working directory to the flutter directory
 PUSHD %flutter_root%
 REM IF doesn't have an "or". Instead, just use GOTO