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

Review URL: https://codereview.chromium.org//133213011

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@32267 260f80e4-7a28-3924-810f-c04153c831b5

computmaxer/pub,cjkao/dart-pub,dart-lang/pub,dart-lang/pub,maxwellpeterson-wf/pub