Prepares widget specifications for the current run-time context.
The functionality has moved to the FieldInfo class. This function is kept as a backwards-compatibility layer. See http://drupal.org/node/1915646.
FieldInfo::prepareInstanceWidget()
function _field_info_prepare_instance_widget($field, $widget) {
$cache = _field_info_field_cache();
return $cache
->prepareInstanceWidget($widget, $field['type']);
}