com.sun.tools.javac.v8.code.ClassReader.list shows up hot in profiles of large
javac runs on non-core classes. Discussion with Neal.Gafter@sfbay indicates that
this routine is being called once per package and, each time, iterates through
(for example) rt.jar. Instead it should iterate once through rt.jar and remember
all packages found. This may improve compilation time significantly since it
appears that this accounts for the bulk of compiled code execution time
(according to runs of javac with -J-Xprof, the HotSpot internal profiler.)