To flatten: 1.
npm install
./node_modules/.bin/truffle-flattener contracts/EternalStorageProxyForStormMultisender.sol > flats/EternalStorageProxyForStormMultisender_flat.sol
./node_modules/.bin/truffle-flattener contracts/multisender/UpgradebleStormSender.sol > flats/UpgradebleStormSender_flat.sol