Update the polyfills; fix a type generation error.

[bicknellr]

• Only modify customElements at load if it's a CustomElementRegistry. webcomponents/polyfills#235, polyfills release, and a version bump here.
• Update local dependencies in webcomponentsjs. webcomponents/polyfills#244, polyfills release, and a version bump here.

base = class extends base {}; causes a type declare class base extends base {} to be emitted but this fails to verify in the next step. Also, I didn't want to ignore everything called base so I renamed it.

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Includes cherry-picked commits from #5607.

[bicknellr]

Lerna didn't update the dependencies in webcomponentsjs during the last publish because the package itself didn't have changes. Fixing in webcomponents/polyfills#244 and then I'll bump here.

[stale]

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