Prepare to release v0.8.1 (dart-lang/source_gen#328)

Author: kevmoo

pkgs/source_gen/pubspec.yaml

@@ -1,5 +1,5 @@
 name: source_gen
-version: 0.8.1-dev
+version: 0.8.1
 author: Dart Team <[email protected]>
 description: Automated source code generation for Dart.
 homepage: https://github.com/dart-lang/source_gen