assetServerURL(): ResolvedAssetSource {
    invariant(!!this.serverUrl, 'need server to load from');
    return this.fromSource(
      this.serverUrl + getScaledAssetPath(this.asset) +
      '?platform=' + Platform.OS + '&hash=' + this.asset.hash
    );
  }
Returns an absolute URL which can be used to fetch the asset
from the devserver