How do I use Code Fragment Mode when debugging a Flex app?

No matter what I type in Code Fragment Mode in the expression evaluator in Flex, all I get is the first line evaluated, and the rest parrotted back to me.  For example, if I enter this:

var s : String = "";
for (var i : int = 0; i < 10; i++)
{
  s += "a";
}

What I'll get in my result is this:

"";
for (var i : int = 0; i < 10; i++)
{
  s += "a";
}

I've read the other threads here that mention CFM, and tried things like putting "s=s" at the end of my fragment, but nothing seems to work.  What am I doing wrong?