Avoid "cannot create a Trace from null" errors in pub.

Avoid "cannot create a Trace from null" errors in pub.

These errors cover up other errors. They were caused because dart:io
produces asynchronous errors without associated stack traces, and when
running without --trace or --verbose pub doesn't add its own trace
information to these errors.

R=alanknight@google.com
BUG=16470
Committed: https://code.google.com/p/dart/source/detail?r=32267

[Alan Knight, 2014-02-03 22:51:17]

lgtm

[nweiz, 2014-02-03 23:03:02]

Committed patchset #2 manually as r32267 (presubmit successful).