Deprecate the mirror matchers library (#131)

I cannot find usage. Asserting that an object has a property with a
known name can be accomplished by casting to `dynamic` and attempting to
read from that property. Asserting that an object has a property with a
name that is unknown at compile time has no known use case, if it comes
up it would re rare enough that the code for using mirrors to solve it
can be put into the test.
diff --git a/CHANGELOG.md b/CHANGELOG.md
index f0cbf07..2d42539 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,6 @@
-## 0.12.7-dev
+## 0.12.7
 
+- Deprecate the `mirror_matchers.dart` library.
 - Update minimum Dart SDK to `2.4.0`.
 
 ## 0.12.6
diff --git a/lib/mirror_matchers.dart b/lib/mirror_matchers.dart
index 8a7a95e..72b2096 100644
--- a/lib/mirror_matchers.dart
+++ b/lib/mirror_matchers.dart
@@ -2,6 +2,9 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+@deprecated
+library mirror_matchers;
+
 /// The mirror matchers library provides some additional matchers that
 /// make use of `dart:mirrors`.
 import 'dart:mirrors';
diff --git a/pubspec.yaml b/pubspec.yaml
index f4f951c..d472547 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: matcher
-version: 0.12.7-dev
+version: 0.12.7
 
 description: >-
   Support for specifying test expectations via an extensible Matcher class.
diff --git a/test/mirror_matchers_test.dart b/test/mirror_matchers_test.dart
index c88e168..beff992 100644
--- a/test/mirror_matchers_test.dart
+++ b/test/mirror_matchers_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// ignore_for_file: deprecated_member_use_from_same_package
+
 @TestOn('vm')
 
 import 'package:matcher/mirror_matchers.dart';