formatValues(...keys) {
    // Convert string keys into arrays that `formatWithFallback` expects.
    const keyTuples = keys.map(
      key => Array.isArray(key) ? key : [key, null]
    );
    return this.interactive.then(
      bundles => this.formatWithFallback(
        bundles, contexts.get(bundles[0]), keyTuples, this.valueFromContext
      )
    );
  }
Retrieve translations corresponding to the passed keys.

A generalized version of `Localization.formatValue`.  Keys can either be
simple string identifiers or `[id, args]` arrays.

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

    // ['Hello, Mary!', 'Hello, John!', 'Welcome!']

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

@param   {...(Array | string)} keys
@returns {Promise<Array<string>>}