Cope with absolute file destinations on Windows

It is assumed that absolute file sources should be copied to the root of
the target directory.

On unix-based systems, an absolute path begins with a '/', which is
interpreted as 'start at root' and does the right thing. On Windows,
these paths contain ':' (e.g., C:/Source) which is an invalid path.
Improve diagnostics output if class definitions are missing
Update current directory magic in runtime.js

Previous behaviour caused build problems on Windows. New logic does the
same thing (effectively) as the previous implementation, whilst avoiding
build failures on Windows.