flutter: readd package_config.json (#394888)

      echo "$(git rev-parse HEAD)" > bin/cache/flutter_tools.stamp

      ln -s '${flutterTools}/share/flutter_tools.snapshot' bin/cache/flutter_tools.snapshot

      # Some of flutter_tools's dependencies contain static assets. The

      # application attempts to read its own package_config.json to find these

      # assets at runtime.

      mkdir -p packages/flutter_tools/.dart_tool

      ln -s '${flutterTools.pubcache}/package_config.json' packages/flutter_tools/.dart_tool/package_config.json

      echo -n "${version}" > version

      cat <<EOF > bin/cache/flutter.version.json

      {