Don't print a spurious error message when exiting

The dart:fuchsia.exit() call uses Isolate.current.kill() to exit. This
generates a spurious error in the log. This change suppresses that error
by checking if an exit code has been set. The exit code will only be set
right before Isolate.current.kill() is called.

US-362 #done

Change-Id: I291e9669ac9612b85ecb42731dd3d6af87629137
3 files changed