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

pkg/oauth2/test/utils.dart

Index: pkg/oauth2/test/utils.dart
diff --git a/pkg/oauth2/test/utils.dart b/pkg/oauth2/test/utils.dart
index 2b6c1a7aa33b0cbbc6ef34bf9f8cb26bca480978..0935658979fda7dbf3d16d25e6f96dfeac539a28 100644
--- a/pkg/oauth2/test/utils.dart
+++ b/pkg/oauth2/test/utils.dart
@@ -45,19 +45,6 @@ class ExpectClient extends MockClient {
   }
 }
 
-// TODO(nweiz): remove this once it's built in to unittest
-/// 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 AuthorizationExceptions.
 const isAuthorizationException = const _AuthorizationException();