constructor(element, name, strings) {
        this.dirty = true;
        this.element = element;
        this.name = name;
        this.strings = strings;
        this.parts = [];
        for (let i = 0; i < strings.length - 1; i++) {
            this.parts[i] = this._createPart();
        }
    }
Sets attribute values for AttributeParts, so that the value is only set once
even if there are multiple parts for an attribute.