I presume some clever chap will soon post a script or some such that does what's needed. And of course, I prefer to wait until this technique has been well tested.
Assuming you're running Time Machine (and if not, why not?) you can restore the missing file from the last time it was backed up (back when you were last on 10.7.3) with the following script.
You must be running as an admin, but only because you need root privileges to restore files into /System. If the missing file were one of your own, you could leave out the one use of
sudo.
This script also presumes your TM backup volume is mounted. I don't have a Time Capsule, and can't test what happens in that case. (It's my understanding, although I could be wrong, that with a Time Capsule the backup volume is mounted only temporarily, when TM needs it.)
# First, identify the file to be restored
FYL=/System/Library/CoreServices/Dock.app/Contents/Resources/url.png
# Don't bother restoring it if it's already present
if [[ -e "$FYL" ]]; then echo $(basename "$FYL") is already installed
else
# Get the name of your machine's TM backup
TM=$(tmutil machinedirectory)
# Get the name of your machine's boot volume
DSK=$(find -x /Volumes -type l -exec basename {} \;)
# Find the last time that file was backed up from your boot volume
SRC=$(ls "$TM"/*/"$DSK$FYL" | tail -1)
# restore it
sudo tmutil restore "$SRC" "$FYL"
# In this special case, because the restored file needs to be noticed by Dock,
# get Dock's attention
osascript -e 'tell application "Dock" to quit'
fi