get _allImageryLayersReady() {
    if (!this._hasImagery) {
      return true;
    }
    const imageryLayers = this._model.imageryLayers;
    const length = imageryLayers.length;
    for (let i = 0; i < length; i++) {
      const imageryLayer = imageryLayers.get(i);
      if (!imageryLayer.ready) {
        return false;
      }
    }
    return true;
  }
Returns whether all imagery layers that are associated with the
model are <code>ready</code>.

If the model does not have imagery, then this always returns
<code>true</code>. Otherwise, it returns whether each imagery
layer is <code>ready</code>.

@private