This is more-or-less a revert of https://github.com/flutter/engine/pull/14011
This code never ended up being used outside of tests, and it's not how we handle asset loading at this point anyway.
I was hopeful we could kill off all runtime dependencies on Dart in `FML` when looking at this, but it looks like trace_event.h still wants to import dart_api_tools.h for some Dart enum types. This may or may not matter if we ever want to build FML for web/wasm. /cc @eyebrowsoffire. If we really need to do that, we can refactor the trace event stuff so that it has a web and Dart implementation that's selected at build time.
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