Merge null_safety branch into master (#125)

Migrates this package to null safety and prepares for a null safety pre-release, once we have landed the package internally and in the SDK to verify its accuracy. 
53 files changed
tree: 8fd71e25b3f85ae053f05f6943fbf0f71e8c4ac4
  1. .gitignore
  2. .test_config
  3. .travis.yml
  4. AUTHORS
  5. CHANGELOG.md
  6. CONTRIBUTING.md
  7. LICENSE
  8. README.md
  9. analysis_options.yaml
  10. lib/
  11. pubspec.yaml
  12. test/
README.md

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