It's no longer needed with the latest Dart SDK roll.

* Make ProcessSignals portable

This removes the need to wrap unsupported signals with in `if (!platform.isWindows) ..`.

It also allows us to implement a work around for breaking the Windows console when flutter is exited with Ctrl+C.

* review comments

* adding tests

* add license header

This ensures that accidental usages of dart:io's
file API don't creep in over time.