RFR 8248157: JShell: throws AssertionError when creating a variable with some unicode chars

[Robert Field]

Unicode in JShell is a broader issue, as covered in the issues I've just 
added as falling out from this:

    8249197: JShell: variable declaration with unicode type name gets 
garbled result
https://bugs.openjdk.java.net/browse/JDK-8249197

    8249199: JShell: Consistent representation of unicode
https://bugs.openjdk.java.net/browse/JDK-8249199

This fix is for the narrow problem specified in the bug report.

Webrev:

http://cr.openjdk.java.net/~rfield/8248157v0.webrev/

Thanks,
Robert