Prepare to publish for stable null safety (dart-lang/stream_channel#72)

pkgs/stream_channel/CHANGELOG.md

diff --git a/pkgs/stream_channel/CHANGELOG.md b/pkgs/stream_channel/CHANGELOG.md
index ea31ffe..2bb6c1f 100644
--- a/pkgs/stream_channel/CHANGELOG.md
+++ b/pkgs/stream_channel/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 2.1.0
+
+* Stable release for null safety.
+
 ## 2.1.0-nullsafety.3
 
 * Update SDK constraints to `>=2.12.0-0 <3.0.0` based on beta release

pkgs/stream_channel/pubspec.yaml

diff --git a/pkgs/stream_channel/pubspec.yaml b/pkgs/stream_channel/pubspec.yaml
index d11a356..e7f6c7a 100644
--- a/pkgs/stream_channel/pubspec.yaml
+++ b/pkgs/stream_channel/pubspec.yaml
@@ -1,5 +1,5 @@
 name: stream_channel
-version: 2.1.0-nullsafety.3
+version: 2.1.0
 
 description: >-
   An abstraction for two-way communication channels based on the Dart Stream
@@ -10,7 +10,7 @@
   sdk: ">=2.12.0-0 <3.0.0"
 
 dependencies:
-  async: '>=2.5.0-nullsafety <2.5.0'
+  async: ^2.5.0
 
 dev_dependencies:
   pedantic: ^1.10.0-nullsafety