As far as I know Eclipse and Flash Builder do not support virtual word wrap as a native feature. Here is the long standing Eclipse bug request:
For Flex Builder 3 there used to be a plug-in by Ahtik that worked but their site seems to be down.
Someone from the Flash Builder team may be able to provide more info or point you to an existing enhancement request that you can vote for.
Sorry, I don't have anything to add but I'll ask around. (I didn't even know that anyoned liked running word wrap for code!)
A related issue is that Builder should have a "line width" preference and respect it (i.e. insert newlines to wrap) for generated code, but that's separate from this as I understand the question.