Steps to repro:

1. Open attached project;
2. In the Project folder go to Assets -> Materials;
3. Open 'StandardTest' shader;

4. Notice that in the Inspector warning "multi_compile keywords must start with a letter or underscore, ignoring '//' " is displayed.

5. Delete comment in line 88;
6. Save shader file;

7. Notice that shader compiled successfully. See attached video for clarity.

Expected result:
Shader compiler should treat '//' symbols coming after #pragma directive as a start of a comment, not as an input variant.
In this case, shader should compile with a line like that: ' #pragma multi_compile _ LOD_FADE_CROSSFADE // uncomment to enable dithering LOD crossfade'.

Notes:
- Reproducible on Windows 10 and OSX 10.12.3;
- Reproducible in 2017.1.0a5, 5.6.0f2, 5.5.2p1.