message: Properly handle negation in IsOk matcher
diff: @@ -115,7 +115,8 @@ std::string StatusToString(const util::StatusOr<T>& s) {
  // Matches a util::StatusOk() value.
  // This is better than EXPECT_TRUE(status.ok())
  // because the error message is a part of the failure messsage.
  -MATCHER(IsOk, "is a Status with an OK value") {
  +MATCHER(IsOk,
  + absl::StrCat(negation ? "isn't" : "is", " a Status with an OK value")) {
  if (arg.ok()) {
  return true;
  }
  