@Override
    public T getVisibleItem(int indexOnPage) {
        checkRowBounds(indexOnPage);
        return presenter.getVisibleItem(indexOnPage);
    }
Get the key for the specified value. If a keyProvider is not specified or the value is null,
the value is returned. If the key provider is specified, it is used to get the key from
the value.

@param value the value
@return the key
stroom-core-client-widget/src/main/java/com/google/gwt/user/cellview/client/AbstractHasData.java