message: Add assert when updating test expectations
diff: @@ -244,6 +244,7 @@ void UpdateTestExpectation(const std::string& filename, const std::string& expec
  // Replace expectation
  auto it = str.find(expectation);
  + assert(it != std::string::npos);
  str.replace(it, expectation.size(), actual);
  // Write it back out.
  