Allow internal usage of SIGQUIT signal listening.

Allow internal usage of SIGQUIT signal listening.

BUG=
R=sgjesse@google.com

Committed: https://code.google.com/p/dart/source/detail?r=35616

[Anders Johnsen, 2014-04-29 19:37:09]

This will allow the following, until we get a way to invoke it through runtime
calls:

import 'dart:io';
import 'dart:mirrors';

void main() {
  var io = currentMirrorSystem().findLibrary(const Symbol('dart.io'));
  var c = MirrorSystem.getSymbol('_ProcessUtils', io);
  var m = MirrorSystem.getSymbol('_watchSignalInternal', io);
  var processUtils = io.declarations[c];

  processUtils.invoke(m, [ProcessSignal.SIGQUIT]).reflectee.listen(print);
}

[Søren Gjesse, 2014-05-01 07:21:54]

lgtm

[Anders Johnsen, 2014-05-01 08:01:18]

Committed patchset #1 manually as r35616 (presubmit successful).