Fix build for throwsStateError: remove duplicate definitions in other tests.

pkg/http/test/utils.dart

Index: pkg/http/test/utils.dart
diff --git a/pkg/http/test/utils.dart b/pkg/http/test/utils.dart
index 5fc267e947cf802566226e4417392294f82ae1ea..b135d715c14f1bc3a1576e65d19dd3533d65227a 100644
--- a/pkg/http/test/utils.dart
+++ b/pkg/http/test/utils.dart
@@ -138,19 +138,6 @@ class _Parse extends BaseMatcher {
   }
 }
 
-// TODO(nweiz): remove this once it's built in to unittest (issue 7922).
-/// 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 HttpExceptions.
 const isHttpException = const _HttpException();