Remove unneeded dart:async imports

Since Dart 2.1, Future and Stream have been exported from dart:core

Alternatively, if for some reason this package must continue to support
Dart 2.0, we can make an exception for these internally.
5 files changed
tree: bcd35a5acdaf1b7a823e436cbeaa1c9bca0d3210
  1. pkgs/