>>>
experimentalBootstrap = document
    .querySelectorAll('link')
    .any((link) => link.attributes['rel'] ==
    'import' && link.attributes['href'] == POLYMER_EXPERIMENTAL_HTML);
<<<
experimentalBootstrap = document.querySelectorAll('link').any((link) =>
    link.attributes['rel'] == 'import' &&
    link.attributes['href'] == POLYMER_EXPERIMENTAL_HTML);