Closure can return an output:

> output_info
>   The value of this parameter indicates the kind of information that you want from the compiler.
>   There are four possible kinds of output: compiled_code, warnings, errors, and statistics. This 
>   example uses the value compiled_code, which tells the Closure Compiler service to output the 
>   compressed version of the JavaScript it receives in the request.

In here it's used to output 'warnings' and/or 'statistics'.