SI-7427 stop crashing under -Ydebug.

Change "dumpClassesAndAbort" to "devWarning". You can witness
it happen like so.
```
  % scalac test/files/pos/t7427.scala -Ydebug -Xdev
  ...
  [running phase cleanup on t7427.scala]
  [running phase icode on t7427.scala]
  warning: !!! PJUMP(method matchEnd4)/GenICode$PJUMP
    is not a control flow instruction
  warning: !!! PJUMP(method case6)/GenICode$PJUMP
    is not a control flow instruction
  [running phase inliner on t7427.scala]
  [running phase inlinehandlers on t7427.scala]
```
Having now lived with this for months, I have no ambition to unravel the
actual problem, I just want it to stop crashing.