Some of the code in the standard library is created by code generation based on templates. For example, many Array methods need to be implemented separately for Array, ByteArray, ShortArray, IntArray, etc.
To run the code generator from a kotlin checkout
cd libraries/tools/kotlin-stdlib-gen mvn compile exec:java
This then runs the GenerateStandardLib.kt script to create the source from the files for java.lang.Iterable and java.util.Collection etc.