This enables running other C/C++ code before the Dart main function
without having to implement the full embedder API by including
main_lib and friends in a build but excluding `main.cc`.
TEST=ci build + flutter bot
Change-Id: I5739eb21fa6918b0a2c0f33f7d97dad73c5cf0b1
Cq-Include-Trybots: luci.dart.try:flutter-engine-linux-try,flutter-linux-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/278518
Commit-Queue: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>
Daco Harkes