Truffle Flattener concats solidity files developed under Truffle with all of their dependencies.
This tool helps you to verify contracts developed with Truffle on Etherscan, or debugging them on Remix, by merging your files and their dependencies in the right order.
npm install truffle-flattener -g
Just intall it with npm in your truffle project and run
truffle-flattener <solidity-files>.
Why not Solidity Flattener?
This project is a Truffle specific
reimplementation of Solidity Flattener. By being closely coupled to Truffle it
can take advantage of its dependencies resolution logic making --solc-paths a
thing of the past. It also supports flattening more than one file at once,
concatenating everything in the right order, whithout duplicating any file.