dexdump doesn't include all dex files for multidex apps. here's a bash hack that does.

multidexdump.sh

#!/bin/bash
# Usage: multidexdump apk [dexdump args]
# Example utility:
#   multidexdump myapp.apk | grep 'Class descriptor'
temp=$(mktemp -d -t "dex")
unzip -qq $1 -d $temp

# Toss apk argument.
shift

for dex in $(ls $temp/*.dex); do
    dexdump "$@" $dex 
done

rm -rf $temp