Those little curly punctuation marks can cause controversy. Some folks are entirely end-of-the-line adherents, others are stridently next-liners. The templates and code completion within Flash Builder follow the next-line model. For some end-of-line folks this can grate like fingernails on a chalkboard.
The default braces are changed within Flash Builder at two places, one for file templates and the other for code block templates. To change the file templates open the preferences for Flash Builder. Go to the Flash Builder->File Templates section and there is a list of all the file templates used in Flash Builder. To edit the code block templates, go to Flash Builder->Editor->Code Templates section.