Blocked by #43642
Part of #16311

Once hb-subset is available from engine builds, we should consume it, we should parse out the constants from the Dart kernel file and use hb-subset to create a new, smaller font based on those constants. We'll want to add an annotation to the relevant classes/constants to make this something extensible beyond the Material and Cupertino icon fonts.

There is an example of parsing the constants from a kernel file in //third_party/dart/pkg/kernel/bin/dump.dart from the Dart SDK.

Related: b/127886310

/cc @willlarche @mehmetf @aam @Hixie