A new Flutter package project.
Import the Baseplate class. You can then use the helper functions to build responsive layouts easily.
I've included lots of breakpoints to give as much freedom as possible. You don't have to use all. Baseplate operates on a "up to" principle. If the screen width is less than or equal to your breakpoint (and there isn't one smaller) that's the one that will be used.
It's a 12 column layout, similar to Bootstrap on the web.