Make completion tests handle exceptions during getSuggestions().

Make completion tests handle exceptions during getSuggestions().

Previously, if an exception occurred during getSuggestions(), or the
future returned by it completed with an error, the completion test
would exit without calling tearDown().

R=brianwilkerson@google.com

Committed: https://code.google.com/p/dart/source/detail?r=42217

[Paul Berry, 2014-12-09 16:59:30]

paulberry@google.com changed reviewers:
+ brianwilkerson@google.com

[Brian Wilkerson, 2014-12-09 19:31:09]

LGTM

[Paul Berry, 2014-12-09 19:34:52]

Committed patchset #1 (id:1) manually as 42217 (presubmit successful).