Steps to reproduce:
1. Create new Unity project
2. Create new Assembly Definition in Project window
3. Deselect asset
4. Rename the created Assembly Definition

Expected results:
Asmdef file name in Project window and Inspector ‘Name’ field match
Actual results:
Asmdef file name in Project window is changed and Inspector ‘Name’ field remains unchanged

Reproduced in: 2018.2.0a1, 2018.1.0b3, 2017.3.0p3, 2017.3.0b1

Note: Changing ‘Name’ value in Inspector does not change the file name either

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By design:
This works the same way as when renaming MonoBehavior scripts. We create the MonoBehaviour script with the same name as the filename, but we do not rename the MonoBehaviour class when renaming the file.