Reproduction steps:

1. Open "ScriptedImportedTest2017.3.zip" project
2. Compare "CubeImporter" and "FooAssetImporter"
3. See the created Assets by the importers

Expected Result: "SetMainAsset" sets the Scriptable Object as the main asset
Actual Result: "SetMainAsset" doesn't set the Scriptable Object as the main asset

Fixed in: 2018.3.0a8

Reproduced with: 2018.1.0a6, 2017.3.0f1, 2017.2.0p4, 2017.1.2p3

Note:
Change these functions from the importers for 2017.2 and below
"AddObjectToAsset" -> "AddSubAsset"
"SetMainObject" -> "SetMainAsset"

From 2017.2 and down it's broken not only with ScriptableObjects