Don't throw an error if a scheduled process exits early.

Don't throw an error if a scheduled process exits early.

Early exits are expected in some cases, since a process's stdout and
stderr streams can close later than the process itself exits. Throwing
an error caused some race conditions. Instead, ScheduledProcess now
prints additional debug info if an error occurs when a test exited
earlier than expected.

R=rnystrom@google.com
BUG=15654
Committed: https://code.google.com/p/dart/source/detail?r=31686

[Bob Nystrom, 2014-01-09 22:51:14]

lgtm

[nweiz, 2014-01-10 01:32:36]

Committed patchset #1 manually as r31686 (presubmit successful).