Fix debugger test.

Fix debugger test.

1. Fix JSON import.

2. Add exit code check to catch such errors in the future.

3. Only kill the debugee on errors. If there are no errors
   it will die on its own with exit code 0. If we kill it
   explicitly it can get killed (and give exit code != 0)
   before it has the chance to exit on its own.

R=sgjesse@google.com,tball@google.com,hausner@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=17889

[Søren Gjesse, 2013-01-31 09:08:50]

lgtm

[Tom Ball, 2013-01-31 14:59:26]

lgtm

https://codereview.chromium.org/12086080/diff/1/tests/standalone/debugger/deb...
File tests/standalone/debugger/debug_lib.dart (right):

https://codereview.chromium.org/12086080/diff/1/tests/standalone/debugger/deb...
tests/standalone/debugger/debug_lib.dart:433: close(killDebugee: true);
Spelling: should be "debuggee", to match "debugger".  Of course there's no such
word as "debuggee", but when has that stopped a software engineer? :-)