Reproduction steps:
1. Open attached project "BugReport_TextureImporter_StreamingMipmapsPriority.zip";
2. Select Assets/Streaming_3;
3. Notice in Inspector it displays "Streaming Mipmaps Priority = 3";
4. Press in main menu "BugReport > Print Streaming Mipmaps Info";
5. Observe the result in the console.

Expected results: TextureImporter.streamingMipmapsPriority returns the correct value, which is set on the object.
Actual results: In Console window, the printed streamingMipmapsPriority is always 1.

Reproduced on: 2018.2.0a1-2018.2.0a3