support the latest pkg:build_runner (#69)

diff --git a/pubspec.yaml b/pubspec.yaml
index 7ca4544..f7f659d 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: async
-version: 2.0.8
+version: 2.0.9-dev
 
 description: Utility functions and classes related to the 'dart:async' library.
 author: Dart Team <misc@dartlang.org>
@@ -16,6 +16,6 @@
   stack_trace: ^1.0.0
   test: ^1.0.0
   # For building and testing with DDC
-  build_runner: ^0.9.0
+  build_runner: ^1.0.0
   build_web_compilers: ^0.4.0
   build_test: ^0.10.1