Adds the ability to pass engine switches temporarily at runtime via
environment variables. Uses the same approach recently added to
Windows and macOS.
This is enabled only for debug/profile to avoid potential issues with
tampering with released applications, but if there is a need for that in
the future it could be added (potentially with a whitelist, as is
currently used for Dart VM flags).
Fixesflutter/flutter#60393
Simplify FlDartProject by removing the path arguments - we'll assume that it's
in the standard bundle layout and add new methods later if we need to support
other cases.
