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)/scala.tools.nsc.backend.icode.GenICode$PJUMP is not a control flow instruction
  warning: !!! PJUMP(method case6)/scala.tools.nsc.backend.icode.GenICode$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.
Merge pull request #2565 from paulp/issue/7427

SI-7427 stop crashing under -Ydebug.
An attempt to make tests deterministic.

The nondeterminism presently showing itself in
presentation/implicit-member is a consequence of the
presentation compiler tests relying on details of the
behavior of toString calls. We need to stomp this out,
but it will take a while. Based on the check file
changes enclosed with this commit, this will suffice
for the presentation compiler tests. A broader assault
will have to take place, but not yet.
Merge pull request #2566 from paulp/pr/nondeterminism

An attempt to make tests deterministic.