Prepare for the 2.11 dev SDKs (#138)

Bump the upper bound to allow 2.10 stable and 2.11.0 dev SDK versions.
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3ca7f6c..7edafcd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 2.5.0-nullsafety.1
+
+* Allow 2.10 stable and 2.11.0 dev SDK versions.
+
 ## 2.5.0-nullsafety
 
 * Migrate this package to null safety.
diff --git a/pubspec.yaml b/pubspec.yaml
index 0ff5f62..cc20a25 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,12 +1,12 @@
 name: async
-version: 2.5.0-nullsafety
+version: 2.5.0-nullsafety.1
 
 description: Utility functions and classes related to the 'dart:async' library.
 homepage: https://www.github.com/dart-lang/async
 
 environment:
   # This must remain a tight constraint until nnbd is stable
-  sdk: '>=2.10.0-0 <2.10.0'
+  sdk: '>=2.10.0-0 <2.11.0'
 
 dependencies:
   collection: '>=1.15.0-nullsafety <1.15.0'