tree bcd35a5acdaf1b7a823e436cbeaa1c9bca0d3210
parent c2e8ce814288b8495407ca8239e848749d3a1646
author Mike Fairhurst <mfairhurst@google.com> 1600805503 -0700
committer Mike Fairhurst <mfairhurst@google.com> 1601322312 -0700

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.