Prepare to release v0.12.3 (#93)

diff --git a/pubspec.yaml b/pubspec.yaml
index 268d102..2fa2ba0 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: matcher
-version: 0.12.3-dev
+version: 0.12.3
 author: Dart Team <misc@dartlang.org>
 description: Support for specifying test expectations
 homepage: https://github.com/dart-lang/matcher