Dart plugin registration generation depends on the target main file
being set as a define so that it's available in the environment, but the
resident runner wasn't setting it. That worked for initial builds
because of the re-entrant call to `assemble` (which does set it), but
caused Hot Restart to always re-generate the wrapper using
lib/main.dart.

This ensures that the define is set in that codepath as well.

Fixes https://github.com/flutter/flutter/issues/91535