Validate the directives that appear at a property field.

def validate_property_directives(directives):
    """Validate the directives that appear at a property field."""
    for directive_name in six.iterkeys(directives):
        if directive_name in VERTEX_ONLY_DIRECTIVES:
            raise GraphQLCompilationError(
                u'Found vertex-only directive {} set on property.'.format(directive_name))