Function preview 

pub fn preview(
    w: &BundleWidget,
    declared: &BTreeMap<String, String>,
    installed: &[WidgetPackageInfo],
    describe_signature: impl Fn(&str, Option<&str>) -> WidgetSignatureInfo,
) -> BundleImportWidgetPreview

Checks w as the widget code would, against what the bundle declares and what is installed here.