Since 1.3.4 update Math Library for Unity includes all source code.

It was popular demand on the Unity Forums to release library source files. After some considerations we decided it is the move in the right direction and sources were added in addition to the compiled dll. Thus it’s possible now to trim the library size (important for the web and mobiles) and learn how parts of the library work. As the dll is still included one have to remove it from the project and exctract source from the archive.