No AssetBundle.mainAsset with BuildingAssetBundles in 5.x ?

Hi folks.

In Unity 4.x we used BuildPipeline.BuildAssetBundle to build an AssetBundle.
The first parameter was the mainAsset:

mainAsset Lets you specify a specific object that can be conveniently retrieved using AssetBundle.mainAsset.

But as BuildAssetBundle is now marked obsolete we use BuildPipeline.BuildAssetBundles in Unity 5.x.
But this function has no parameter mainAsset. And I can’t find a “Set main asset” option in the AssetBundle authoring tools either.

Question: Is there another way to set the mainAsset parameter for each AssetBundle in Unity 5.x?

public static object getMainAsset(AssetBundle assetBundle){
return assetBundle.LoadAsset(assetBundle.GetAllAssetNames()[0]);

I have the same problem! Now I do a mapping for mainAsset.