Prepare to release 0.12.0-alpha.0.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//944683002
diff --git a/pubspec.yaml b/pubspec.yaml
index aabf395..05ec1d6 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,14 +1,9 @@
 name: matcher
-version: 0.12.0-dev
+version: 0.12.0-alpha.0
 author: Dart Team <misc@dartlang.org>
 description: Support for specifying test expectations
 homepage: https://github.com/dart-lang/matcher
 environment:
   sdk: '>=1.0.0 <2.0.0'
 dev_dependencies:
-  # Using a git dependency to align with the unreleased version of unittest
-  # that is compatible with matcher 0.12.0
-  unittest:
-    git:
-      ref: master
-      url: https://github.com/dart-lang/unittest
+  unittest: '>=0.12.0-alpha.0 <0.13.0'