code: public long getLong(String keyStr) {
      SoyData valueData = get(keyStr);
      if (valueData == null) {
        throw new IllegalArgumentException("Missing key: " + keyStr);
      }
      return valueData.longValue();
    }
docs: Precondition: The specified key string is the path to a long. Gets the long at the specified
  key string.
  
  @param keyStr One or more map keys and/or list indices (separated by '.' if multiple parts).
  Indicates the path to the location within this data tree.
  @return The long at the specified key string.
  @throws IllegalArgumentException If no data is stored at the specified key.