Clean up `CancelableOperation`. (#204)

* Clean up `CancelableOperation`.

Some clean-ups and simplifications of the code.
Should not change any behavior, but differences in future chaining
can potentially change timing.

But don't change `Future` to `Future<void>` today.
3 files changed
tree: 0a1c61e89eb5df848a88e57566fccfb41270f884
  1. .github/
  2. lib/
  3. test/
  4. .gitignore
  5. analysis_options.yaml
  6. AUTHORS
  7. CHANGELOG.md
  8. CONTRIBUTING.md
  9. LICENSE
  10. pubspec.yaml
  11. README.md
README.md

Contains utility classes in the style of dart:async to work with asynchronous computations.