Remove unnecessary dart:async import.
With the version constraint excluding 2.0, the import is not necessary.
diff --git a/lib/pedantic.dart b/lib/pedantic.dart
index 5276dc9..7998a89 100644
--- a/lib/pedantic.dart
+++ b/lib/pedantic.dart
@@ -2,8 +2,6 @@
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
-import 'dart:async' show Future;
-
/// Indicates to tools that [future] is intentionally not `await`-ed.
///
/// In an `async` context, it is normally expected that all [Future]s are
