Flattener hangs due to css import rule

[plinki]

Contracts may use the css @import rule when constructing svgs, the flattener mistakes these for solidity imports and hangs due to being unable to find the file, which doesn't exist

string[0] memory parts;
parts[0] = '<svg xmlns="http://www.w3.org/2000/svg"><style>@import url("https://use.typekit.net/nln0qsp.css");</style></svg>';