The extension mechanism that was previously discussed on this page—native extensions—was removed in Dart 2.15.
If you need to call existing code written in C or C++, see the FFI documentation.
A mechanism that's similar to native extensions—the Dart Embedding API—is supported when the Dart VM is embedded as a library into another application. For examples of how to use the Dart Embedding API, see these examples maintained by the community.