Panic when copying private properties.
Panic when copying private properties.

When CopyProperties comes across private properties, it silently ignores
them. This is error-prone and can result in a bug very hard to debug if
the developer is unaware of the behavior.

Change-Id: Id6a0752e384ec7fed35728c1e87dbfa95fea84f2

google/blueprint,danw/blueprint,danw/blueprint,google/blueprint