To reproduce:

1. Download attached project "excel-test.zip" and open in Unity
2. Observe Console and you should see two errors:

"Assets\Editor\ExcelDataFoo.cs(10,11): error CS0012: The type 'IDataReader' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Data, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'."

"Assets\Editor\ExcelDataFoo.cs(10,11): error CS0012: The type 'IDataRecord' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Data, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'."

Notes:
- This issue appears both on Windows and OSX
- Adding reference does not fix this issue
- This is the version of ExcelDataReader 3.4.0 for .NET Standard 2.0 from Nuget
- The same issue happens with the version targeting .NET Standard 1.3

Reproduced on Unity 2018.1.9f2, 2018.2.6f1 and 2018.3.0a10