def dependents_of(self, address):
    """Returns the addresses of the targets that depend on the target at `address`.

    This method asserts that the address given is actually in the BuildGraph.

    :API: public
    """
    assert address in self._target_by_address, (
      'Cannot retrieve dependents of {address} because it is not in the BuildGraph.'
      .format(address=address)
    )
    return self._target_dependees_by_address[address]