Const Finder

This program uses package:kernel from the Dart SDK in //third_party.

A snapshot is created via the build rules in BUILD.gn. This is then vended to the Flutter tool, which uses it to find const creations of IconData classes. The information from this can then be passed to the font-subset tool to create a smaller icon font file specific to the application.

Once flutter/flutter#47162 is resolved, this package should be moved to the flutter tool.