Having some assertion set is ok, but I think adding a dependency to Guava only for these assertions is overkill. We should add our own assertion set to msgpack-core.

Preconditions.java 
https://code.google.com/p/guava-libraries/source/browse/guava/src/com/google/common/base/Preconditions.java?r=806967c3660d231e62a52674703acb51ec4e6623