How/why should it be in the AUR?
There is no need to install this to use it. The recommended usage is to link to their js compiler in your webpage.
Do you want to "precompile" pages on your own computer? If so, then you could make a PKGBUILD for this - but there is not installation method provided by the authors and all the files within the various bits' directory structure refers to other parts by relative paths. This means you'd have to use the git svn command to get that directory tree, then install the whole thing as is to somewhere like /usr/lib/traceur-compiler then create a link in /usr/bin/traceur to /usr/lib/traceur-compiler/traceurc to be able to actually run it.