Update SDK contraint lower bound to 2.0.0-dev.15.0
diff --git a/pubspec.yaml b/pubspec.yaml
index deaf2c3..bfc4171 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -4,7 +4,7 @@
 description: Utility functions and classes related to the 'dart:async' library.
 homepage: https://www.github.com/dart-lang/async
 environment:
-  sdk: ">=2.0.0-dev <2.0.0"
+  sdk: ">=2.0.0-dev.15.0 <2.0.0"
 dependencies:
   collection: "^1.5.0"
 dev_dependencies: