class file of a Symbol (compiler question)

I'm trying to get the class file that a Symbol was loaded from in a compiler 
plugin.  Specifically, inside the run method of a Phase I am doing something 
like:

for(unit <- currentRun.units; sym <- unit.depends)
  println(sym.toplevelClass.asInstanceOf[ClassSymbol].classFile)

but this always seems to print null.  From what I can tell, 
ClassSymbol.classFile is not set anywhere in the compiler.  It looks to me 
like this used to happen in ClassfileParser.parse around line 66, but was 
commented out in changeset 12705.

So, instead, I have to do something like

val name = sym.fullNameString(File.separatorChar) + 
(if(sym.hasFlag(MODULE)) "$" else "")
    println(classPath.root.find(name, false).classFile)

(as is done in DefaultDocDriver) and this works as expected but is relatively 
slow.

Should ClassSymbol.classFile work the way I think it should or am I doing 
something wrong?

Thanks,
Mark