Emit warning about class literal incompatibility

When using -target 5 or newer, javac generates code that affects class
initialization of class literals differently than older targets:

  http://java.sun.com/j2se/1.5.0/compatibility.html#literal

Unfortunately, many developers are unaware of the information provided
in this page and some assistance from the compiler could be useful.

The compiler should emit a warning about the situation. It is not
immediately apparent how this should be integrated with the existing
-Xlint feature. Should this be turned on with -Xlint:all?