unittest fix: Get rid of the deprecation of IllegalJsRegexException's matcher.

pkg/unittest/test/matchers_test.dart

Index: pkg/unittest/test/matchers_test.dart
diff --git a/pkg/unittest/test/matchers_test.dart b/pkg/unittest/test/matchers_test.dart
index 71cdb6b4394dc603bcbdf33c78a98b7ae0e175cb..b11f7a6dca180aedca1149189b4c381c894b2a5e 100644
--- a/pkg/unittest/test/matchers_test.dart
+++ b/pkg/unittest/test/matchers_test.dart
@@ -158,16 +158,6 @@ void main() {
             "ArgumentError.");
     });
 
-    test('throwsIllegalJSRegExpException', () {
-      shouldPass(() { throw new IllegalJSRegExpException('',''); },
-          throwsIllegalJSRegExpException);
-      shouldFail(() { throw new Exception(); },
-          throwsIllegalJSRegExpException,
-        "Expected: throws an exception which matches IllegalJSRegExpException "
-        "but:  exception <Exception> does not match "
-            "IllegalJSRegExpException.");
-    });
-
     test('throwsRangeError', () {
       shouldPass(() { throw new RangeError(0); },
           throwsRangeError);