Steps to repro:

1. Open attached project and 'testShadow' scene;
2. Launch the scene.

Actual result:
Light ray from spotlight gameobject passes through Speedtrees (see attached actual.png screenshot).
The issue is specific for SpeedTree models.

Expected result:
Light ray should not pass through speedtrees to be consistent with any opaque objects (see attached expected.png screenshot).

Notes:
- Reproducible in 5.5.0b7, 5.4.1f1, 5.3.6p6;
- Reproducible on both OSX 10.12 and Windows 10;
- Not reproducible for any other opaque geometry (simple cubes, spheres etc);
- Not reproducible in Deferred rendering path; happening only in Forward rendering path.