/**
 * Whether an element has actually been upgraded into its custom element class yet. Until then it's a plain element: it
 * has none of the component's properties (`disabled`, `checked`, `size`, `value`...), and assigning one creates an
 * instance property that Lit later re-applies on the element's first update — after it has already read the
 * attributes — silently overriding them.
 *
 * A parent that syncs state onto its children has to skip the ones that aren't upgraded yet, which is the normal state
 * of things when components load lazily (e.g. through the autoloader): every tag is its own `import()`, so a parent and
 * its children register in whatever order the network returns them.
 */
export declare function isUpgraded(element: Element): boolean;