RFR 8198573: JShell: class replace loses instances

[Robert Field]

Possible update false-positives are tested with a bytecode comparison.  
This is thwarted because the continuously growing list of imports are 
put on new lines which changes the line-number table.  With local 
execution this causes loss of data.  Fix by keeping all imports on a 
single line.

Bug:

     https://bugs.openjdk.java.net/browse/JDK-8198573

Webrev:

     http://cr.openjdk.java.net/~rfield/8198573v0.webrev/


Thanks,
Robert