Update the constraint on matcher to hit the published version.

pubspec.yaml

Index: pubspec.yaml
diff --git a/pubspec.yaml b/pubspec.yaml
index 2be4eaeb2c858484f55bc5211614e1eaad04abd9..5de05077fd13cb8fce323982f37da5ee370ff4f0 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -8,11 +8,8 @@ environment:
 dependencies:
   args: '>=0.12.1 <0.13.0'
   stack_trace: '>=1.2.0 <2.0.0'
-
-  # Using the pre-release version of matcher. When published we will go back
-  # using a tight version constraint to ensure that a constraint on unittest
+  # Use a tight version constraint to ensure that a constraint on unittest
   # properly constraints all features it provides.
-  matcher:
-    git: https://github.com/dart-lang/matcher.git
+  matcher: '>=0.12.0-alpha.0 <0.12.1'
 dev_dependencies:
   fake_async: '>=0.1.2 <0.2.0'