function assert_implements_optional(condition, description) {
        if (!condition) {
            throw new OptionalFeatureUnsupportedError(description);
        }
    }
Assert that an optional feature is implemented, based on a 'truthy' condition.

This function should be used to early-exit from tests in which there is
no point continuing without support for an explicitly optional spec or
spec feature. For example:

    assert_implements_optional(video.canPlayType("video/webm"),
                               "webm video playback not supported");

@param {object} condition The truthy value to test
@param {string} [description] Error description for the case that the condition is not truthy.