rmic should support v51 class files. JDK is switching (soon) to use v51 class files by default.

From jdk/src/share/classes/sun/tools/java/RuntimeConstants.java

   int JAVA_MAX_SUPPORTED_VERSION = 50;

50 should be raised to 51.

There are no changes in v51 class files at this point (except for the proposed module-info.class files being developed in Project Jigsaw) so raising the limit is currently a safe operation.