Fuchsia's Dart and Flutter content handlers share the implementation of
dart:io with the command line embedder, but would like to disable
some operations that don't make sense for them, like exiting thoe whole
process, which may be hosting multiple unrelated Isolates.
Change-Id: I825ece49c685471ff85802873b1c0f974c507aed
Reviewed-on: https://dart-review.googlesource.com/3687
Reviewed-by: Ryan Macnak <rmacnak@google.com>