function assert_implements(condition, description) {
        assert(!!condition, "assert_implements", description);
    }
Assert that a 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 a non-optional spec or spec
feature. For example:

    assert_implements(window.Foo, 'Foo is not supported');

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