RFR JDK-8068448 - improve the assert

[Eric Caspole]

Hi everybody,
Here is a patch to try to help debug this problem, by explicitly 
collecting more information including start array and heap size when the 
assert occurs.

  http://cr.openjdk.java.net/~ecaspole/JDK-8068448/00/webrev/

Passes JPRT.
Thanks,
Eric