This is fairly straight forward. The small amount of javascript code calls into a helper method already written in JSJavaScriptEngine.java, so mostly it's just moving this java code to CommandProcessor.java, and reworking it a bit:

From sa.js:
    this.dclass = function(clazz, dir) {
      if (!clazz) {
         writeln("Usage: dumpclass { address | name } [ directory ]");
      } else {
         if (!dir) { dir = "."; }
         dumpClass(clazz, dir);
      }
    }
    registerCommand("dumpclass", "dumpclass { address | name } [ directory ]", "dclass");

From JSJavaScriptEngine.java:
    public Object dumpClass(Object[] args) {
        if (args.length == 0) {
            return Boolean.FALSE;
        }
        Object clazz = args[0];
      if (clazz == null) {
          return Boolean.FALSE;
      }
        InstanceKlass ik = null;
        if (clazz instanceof String) {
            String name = (String) clazz;
            if (name.startsWith("0x")) {
                // treat it as address
                VM vm = VM.getVM();
                Address addr = vm.getDebugger().parseAddress(name);
                Metadata metadata = Metadata.instantiateWrapperFor(addr.addOffsetTo(0));
                if (metadata instanceof InstanceKlass) {
                    ik = (InstanceKlass) metadata;
                } else {
                    return Boolean.FALSE;
                }
            } else {
                ik = SystemDictionaryHelper.findInstanceKlass((String) clazz);
            }
        } else if (clazz instanceof JSJavaClass) {
            JSJavaKlass jk = ((JSJavaClass)clazz).getJSJavaKlass();
            if (jk != null && jk instanceof JSJavaInstanceKlass) {
                ik = ((JSJavaInstanceKlass)jk).getInstanceKlass();
            }
        } else {
            return Boolean.FALSE;
        }

        if (ik == null) return Boolean.FALSE;
        StringBuffer buf = new StringBuffer();
        if (args.length > 1) {
            buf.append(args[1].toString());
        } else {
            buf.append('.');
        }

        buf.append(File.separatorChar);
        buf.append(ik.getName().asString().replace('/', File.separatorChar));
        buf.append(".class");
        String fileName = buf.toString();
        File file = new File(fileName);

        try {
            int index = fileName.lastIndexOf(File.separatorChar);
            File dir = new File(fileName.substring(0, index));
            dir.mkdirs();
            FileOutputStream fos = new FileOutputStream(file);
            ClassWriter cw = new ClassWriter(ik, fos);
            cw.write();
            fos.close();
        } catch (IOException exp) {
            printError(exp.toString(), exp);
            return Boolean.FALSE;
        }

        return Boolean.TRUE;
    }