To reproduce:
1. Open Unity and create new project
2. Edit-> Project Settings -> Player and select .NET 4.6 framework
3. Restart Unity as suggested

Expected: Unity should successfully restart and switch framework to .NET 4.6.
Actual: mono: function mono_unity_capture_memory_snapshot not found
Could not load symbol mono_unity_free_captured_memory_snapshot : (null)
Unity fails to start mono.

Reproduced on: 2017.3.0a6
Not reproducible with 2017.3.0a5
OS: macOS 10.12.6 (16G29)

Not reproducible on Windows 10 64bit

Fixed in: 2017.3.0a7