Helper to add a new attribute to a cache.

def _AddAttributeToCache(self, attribute_name, value, cache):
    """Helper to add a new attribute to a cache."""
    # If there's another value in cache with the same timestamp, the last added
    # one takes precedence. This helps a lot in tests that use FakeTime.
    attribute_list = cache.setdefault(attribute_name, [])
    if attribute_list and attribute_list[-1].age == value.age:
      attribute_list.pop()

    attribute_list.append(value)