static get observedAttributes() {
        // note: piggy backing on this to ensure we're finalized.
        this.finalize();
        const attributes = [];
        // Use forEach so this works even if for/of loops are compiled to for loops
        // expecting arrays
        this._classProperties.forEach((v, p) => {
            const attr = this._attributeNameForProperty(p, v);
            if (attr !== undefined) {
                this._attributeToPropertyMap.set(attr, p);
                attributes.push(attr);
            }
        });
        return attributes;
    }
Returns a list of attributes corresponding to the registered properties.
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