Process object only when the process has been successfully started.
This is less error prone since you can use all the methods on a
Process object as soon as you can get your hands on it. Unfortunately,
this change leads to quite a few changes in test.dart. Some of those
changes will fix bugs as well. We used to have a problem with a
timeout that could fire just in between two steps in a chain of
commands that constitute one test run. This change should address
that issue as well.
BUG=dartbug.com/5305
Review URL: https://codereview.chromium.org//11091070
git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@13729 260f80e4-7a28-3924-810f-c04153c831b5
floitsch@google.com