Add isAssertionError (#118)

Towards https://github.com/dart-lang/test/issues/1094
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 12e0105..eeb000c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 0.12.7
+
+- Add `isAssertionError` matcher.
+
 ## 0.12.6
 
 - Update minimum Dart SDK to `2.2.0`.
diff --git a/lib/src/error_matchers.dart b/lib/src/error_matchers.dart
index 5116975..0153a70 100644
--- a/lib/src/error_matchers.dart
+++ b/lib/src/error_matchers.dart
@@ -40,3 +40,6 @@
 
 /// A matcher for [UnsupportedError].
 const isUnsupportedError = TypeMatcher<UnsupportedError>();
+
+/// A matcher for [AssertionError].
+const isAssertionError = TypeMatcher<AssertionError>();
diff --git a/pubspec.yaml b/pubspec.yaml
index a65ec98..b445f68 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: matcher
-version: 0.12.6
+version: 0.12.7
 
 description: >-
   Support for specifying test expectations via an extensible Matcher class.