Is it possible to extend `dart:async`s Future class

Question

Is it possible to create a custom implementation of the abstract class Future?

Answer 1

Extending or implementing Future is prevented to allow several performance optimizations which wouldn't be possible otherwise.

The package async provides a DelegatingFuture that can be extended instead and which delegates to another future.

For more details see the discussion at https://github.com/dart-lang/sdk/issues/24335