Add matches for StateErrors

pkg/unittest/lib/src/core_matchers.dart

Index: pkg/unittest/lib/src/core_matchers.dart
diff --git a/pkg/unittest/lib/src/core_matchers.dart b/pkg/unittest/lib/src/core_matchers.dart
index fbf146560db8ed7fdd1a30a5b2ff3d8ec0b602ca..489fecd1826a51ea1de590960ca223ccc5872dd3 100644
--- a/pkg/unittest/lib/src/core_matchers.dart
+++ b/pkg/unittest/lib/src/core_matchers.dart
@@ -504,6 +504,19 @@ class _UnsupportedError extends TypeMatcher {
   bool matches(item, MatchState matchState) => item is UnsupportedError;
 }
 
+/** A matcher for StateErrors. */
+const isStateError = const _StateError();
+
+/** A matcher for functions that throw StateError. */
+const Matcher throwsStateError =
+    const Throws(isStateError);
+
+class _StateError extends TypeMatcher {
+  const _StateError() : super("StateError");
+  bool matches(item, MatchState matchState) => item is StateError;
+}
+
+
 /** A matcher for Map types. */
 const isMap = const _IsMap();