ListBuffer could reuse List.nil() as the sentinel element

The com.sun.tools.javac.util.ListBuffer uses a newly created List element as a sentinel element (and when an element is appended to the LB, it is set to the sentinel element and a new sentinel element is created). Maybe it could reuse List.nil()? That could save some memory while compiling.

This issue is based on NetBeans bug:
http://netbeans.org/bugzilla/show_bug.cgi?id=223094