Prepare to release v0.3.1 (google#93)

Author: kevmoo

json_serializable/CHANGELOG.md

@@ -1,8 +1,10 @@
-## 0.3.1-dev
+## 0.3.1
 
 * Add a `build.yaml` so the builder can be consumed by users of `build_runner`
   version 0.7.0.
 
+* Now requires a Dart `2.0.0-dev` release.
+
 ## 0.3.0
 
 * **NEW** top-level library `json_serializable.dart`.

json_serializable/pubspec.yaml

@@ -1,5 +1,5 @@
 name: json_serializable
-version: 0.3.1-dev
+version: 0.3.1
 author: Dart Team <misc@dartlang.org>
 description: Generates utilities to aid in serializing to/from JSON.
 homepage: https://github.com/dart-lang/json_serializable