[mb] tests mb/api/java/nio/CharsetMicro/* fail due to sun/nio/cs/ext/Big5 not found

The following JRockit tests fail with java.lang.ClassNotFoundException: sun.nio.cs.ext.Big5:

java.lang.ClassNotFoundException: sun.nio.cs.ext.Big5
mb/api/java/nio/CharsetMicro/decodeISO_8859_15
mb/api/java/nio/CharsetMicro/decodeUS_ASCII
mb/api/java/nio/CharsetMicro/encodeBig5
mb/api/java/nio/CharsetMicro/encodeUS_ASCII
mb/api/java/nio/CharsetMicro/encodeUTF_32

The failure can be reproduced in the latest checkout of both jdk9/hs and jdk9/hs-comp. The failure appears also with b52, but it does not appear with b44.