RFR 8211694: JShell: Redeclared variable should be reset

[Robert Field]

Please review ---

Bug:

     https://bugs.openjdk.java.net/browse/JDK-8211694

Webrev:

     http://cr.openjdk.java.net/~rfield/8211694v0.webrev/

See query to kulla-dev:

http://mail.openjdk.java.net/pipermail/kulla-dev/2018-October/002316.html

Thanks,
Robert