Steps to reproduce:
1. Create new project
2. Go to Libaries/ScriptAssemblies folder
3. Add a file that will get locked in use (like a small executable when run or a Word .doc document)
4. Run it/ Open it
5. In the new project, create a script and force a compile
6. Compiler will try to delete the file in use, perpetually throw an error and get stuck in compilation

Expected: Either the file in use will be ignored or error thrown and compilation stopped
Actual: Compilation tries to delete the file in perpetual limbo where it never ends until the file is deleted and compilation completes

Reproduced on 2017.1.0b1, 2017.1.0f3, 2017.1.0p3, 2017.2.0b6, 2017.3.0a3
Regression introduced in: 2017.1.0a3
Not reproduced on 5.6.3f1, 2017.1.0a1, 2017.1.0a2