allSourcesReady_() {
    let ready = true;
    let source;
    for (let i = 0, ii = this.layers_.length; i < ii; ++i) {
      source = this.layers_[i].getSource();
      if (!source || source.getState() !== 'ready') {
        ready = false;
        break;
      }
    }
    return ready;
  }
Determine if all sources are ready.
@return {boolean} All sources are ready.
@private