formatEntities(keys) {
    return this.interactive.then(
      bundles => this.formatWithFallback(
        bundles, contexts.get(bundles[0]), keys, this.entityFromContext
      )
    );
  }
Format translations into {value, attrs} objects.

This is an internal method used by `LocalizationObserver` instances.  The
fallback logic is the same as in `formatValues` but the argument type is
stricter (an array of arrays) and it returns {value, attrs} objects which
are suitable for the translation of DOM elements.

    document.l10n.formatEntities([j
      ['hello', { who: 'Mary' }],
      ['welcome', undefined]
    ]).then(console.log);

    // [
    //   { value: 'Hello, Mary!', attrs: null },
    //   { value: 'Welcome!', attrs: { title: 'Hello' } }
    // ]

Returns a Promise resolving to an array of the translation strings.

@param   {Array<Array>} keys
@returns {Promise<Array<{value: string, attrs: Object}>>}
@private