Fix unary operator error when revision variable is empty

bin/flutter

@@ -13,7 +13,7 @@ SCRIPT_PATH="$FLUTTER_TOOLS_DIR/bin/flutter_tools.dart"
 DART=dart
 
 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 "$FLUTTER_TOOLS_DIR/pubspec.lock" ]; then
+if [ ! -f "$SNAPSHOT_PATH" ] || [ ! -f "$STAMP_PATH" ] || [ `cat "$STAMP_PATH"` != "$REVISION" ] || [ "$FLUTTER_TOOLS_DIR/pubspec.yaml" -nt "$FLUTTER_TOOLS_DIR/pubspec.lock" ]; then
   echo Updating flutter tool...
   (cd "$FLUTTER_TOOLS_DIR"; pub get)
   $DART --snapshot="$SNAPSHOT_PATH" --package-root="$FLUTTER_TOOLS_DIR/packages" "$SCRIPT_PATH"