Method firstUpdated

packages/reactive-element/src/test/reactive-element_test.ts:1979–1982

(changedProperties: PropertyValues)

      }

      override firstUpdated(changedProperties: PropertyValues) {
        this.changedProperties = changedProperties;
        this.wasFirstUpdated++;
      }
    }
    customElements.define(generateElementName(), E);
    const el = new E();