How to reproduce:
1. Create new Unity project
2. Go to Edit > Project Settings > Player
3. Change "Scripting Runtime Version" setting from Stable(.NET 3.5 Equivalent) to Experimental(.NET 4.6 Equivalent)
4. Import any package (e.g., user-supplied package or Standard Assets)
5. Right click any folder in Project window, then click Export package
6. Observe the "Exporting package" window

Expected result: Exporting package window has a folder tree view and doesn't show copies of folders in the package.
Actual result: Exporting package window does not show folder tree view and shows repetitions of all the folders.

Reproducible with: 2017.3.0a1, 2017.2.0b4, 2017.1.0p2, 2017.1.0f3, 2017.1.0a6
Not reproducible with: 2017.1.0a5 or lower due to "Scripting Runtime Version" setting not being introduced yet

Note: There was an identical issue with package import, that was fixed in 2017.2.0a3

Fixed in 2017.3.0a3, 2017.2.0b6, 2017.1.0p4