Second attempt to fix issue 3402. It turns out that we can't use

Second attempt to fix issue 3402. It turns out that we can't use
$_, because that contains the original script name (in the case
test.py, $_ contains .../test.py when this script is executed.

Using $0 will work fine, as pwd -P will give the absolute
physical path of the dart script.
Committed: https://code.google.com/p/dart/source/detail?r=13030

[ahe, 2012-09-28 17:17:11]

lgtm