The tool has new assets to subset icon fonts, and new functionality is being added to the assets build target to use these tools to remove unneded icons from fonts.

Each platform needs to be wired in to use this - some are easy and will be done in the same pull request that adds functionality to the assets target, but others are blocked or in flux at the moment.

• iOS (blocked by making iOS use assemble completely - [flutter_tools] Migrate xcode_backend.sh to flutter assemble #49842) (fixed in [flutter_tools] wire up tree shake fonts to iOS #50224)
• Android (Font subset in the tool #49737)
• macOS (Font subset in the tool #49737)
• Linux
• Windows
  [ ] Fuchsia
• Web (does not emit a kernel file at present)