I know that android source code is kept here: https://android.googlesource.com/, but I can't see any way to browse an old version of the code.
How would I view, say, dalvik/libcore/concurrent/src/main/java/java/util/concurrent/LinkedBlockingQueue.java in Froyo?
Try clicking on the platform/dalvik repository. Then scroll down to either one of the tags, or else the froyo-release branch (under Branches). Then just keep following the tree links until you get to the right directory. Then click on the the file you're interested in.