How to reproduce:
1. Create a new project
2. Open the Asset Store and search for "PBR Flower Pack"
3. Download and click Import
4. After it finishes extracting the package import "orange_heptica_01.prefab", "orange_heptica_02.prefab", "pink_heptica_02.prefab" (Screenshot below)

Expected result: Selected prefabs are imported
Actual result: Editor crashes

Reproducible on: 2018.3.0a6, 2018.3.0b4, 2019.1.0a3
Not Reproducible on: 2017.4.12f1, 2018.1.9f2, 2018.2.11f1, 2018.3.0a5

Note: If importing only 2 of these prefabs (e.g pink_heptica_02.prefab and orange_heptica_02.prefab) Editor does not crash, but throws an error in the console saying:
Assertion failed on expression: 'saveFromPrefabInstance || saveFromAssetObjects'

Last 5 lines of stack trace:
0x0000000140FBE773 (Unity) PrefabImporter::GenerateAssetData
0x0000000141F3E3C9 (Unity) AssetDatabaseV1::ImportAsset
0x0000000141F50D9D (Unity) AssetDatabaseV1::UpdateAsset
0x0000000141F443A9 (Unity) AssetInterface::ProcessAssetsImplementation
0x0000000141F4F074 (Unity) AssetInterface::StopAssetEditing