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

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 04a071154db854ded6b0f836f9d38034a085eeef..c412450012140c2a662649a142b4650c4a5feec4 100644
--- a/pkg/unittest/lib/src/core_matchers.dart
+++ b/pkg/unittest/lib/src/core_matchers.dart
@@ -483,19 +483,6 @@ class _ArgumentError extends TypeMatcher {
   bool matches(item, MatchState matchState) => item is ArgumentError;
 }
 
-/** A matcher for IllegalJSRegExpExceptions. */
-const isIllegalJSRegExpException = const _IllegalJSRegExpException();
-
-/** A matcher for functions that throw IllegalJSRegExpException. */
-@deprecated
-const Matcher throwsIllegalJSRegExpException =
-    const Throws(isIllegalJSRegExpException);
-
-class _IllegalJSRegExpException extends TypeMatcher {
-  const _IllegalJSRegExpException() : super("IllegalJSRegExpException");
-  bool matches(item, MatchState matchState) => item is IllegalJSRegExpException;
-}
-
 /** A matcher for RangeErrors. */
 const isRangeError = const _RangeError();