diff --git a/lib/src/observable.dart b/lib/src/observable.dart
index 2f6e0f7..0fe6908 100644
--- a/lib/src/observable.dart
+++ b/lib/src/observable.dart
@@ -4,8 +4,6 @@
 
 library observable.src.observable;
 
-import 'dart:async';
-
 import 'package:meta/meta.dart';
 
 import 'change_notifier.dart';
diff --git a/pubspec.yaml b/pubspec.yaml
index 8de42fd..dc3e33b 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -4,7 +4,7 @@
 description: Support for marking objects as observable
 homepage: https://github.com/google/observable
 environment:
-  sdk: '>=2.0.0 <3.0.0'
+  sdk: '>=2.1.0 <3.0.0'
 
 dependencies:
   collection: ^1.11.0