constructor(element, name, strings) {
        super(element, name, strings);
        this.single =
            (strings.length === 2 && strings[0] === '' && strings[1] === '');
    }
Sets attribute values for PropertyParts, so that the value is only set once
even if there are multiple parts for a property.

If an expression controls the whole property value, then the value is simply
assigned to the property under control. If there are string literals or
multiple expressions, then the strings are expressions are interpolated into
a string first.