App Engine maintains the source code repository for its’ APIs at https://code.google.com/p/googleappengine/source/checkout . Using a Subversion client you can check out and review the code – just follow the directions on the page.
If you would prefer to browse the code online, you can go to https://code.google.com/p/googleappengine/source/browse/ . Open up the folders on the left side navigation bar until you get to the file you’re looking for. Most of the time you’ll want the most recent source code available, so browse to the SVN > Trunk folder and go from there.
Need an open source implementation of the whole production server and its environment? Try AppScale, which is a Google-supported project allowing App Engine applications to run on other servers.