Prevent analysis exceptions from crashing the completion engine.

Prevent analysis exceptions from crashing the completion engine.

Prevent CompletionDomainHandler.priorityChanged() from crashing if the
future returned by DartCompletionManager.computeCache() completes with
an error (which can happen in the event that an exception occurred
while analyzing the file in question).

BUG=dartbug.com/23176
R=danrubel@google.com

Committed: https://code.google.com/p/dart/source/detail?r=45142

[Paul Berry, 2015-04-14 13:20:45]

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

[danrubel, 2015-04-14 13:55:54]

lgtm

[Paul Berry, 2015-04-14 16:30:49]

Committed patchset #1 (id:1) manually as 45142 (presubmit successful).

[kevmoo, 2015-04-14 19:21:16]

On 2015/04/14 16:30:49, Paul Berry wrote:
> Committed patchset #1 (id:1) manually as 45142 (presubmit successful).

DBC: if errors are swallowed, should they be logged?

[Paul Berry, 2015-04-14 19:32:59]

On 2015/04/14 19:21:16, kevmoo wrote:
> On 2015/04/14 16:30:49, Paul Berry wrote:
> > Committed patchset #1 (id:1) manually as 45142 (presubmit successful).
> 
> DBC: if errors are swallowed, should they be logged?

At the time the error is detected (during analysis) it is already logged, but I
suspect it is not logged in a sufficiently visible way, and it is on my list to
investigate that further.

In principle we could also log the error here in
CompletionDomainHandler.priorityChanged(), but I suspect that wouldn't gain us
very much more signal-to-noise over the existing logging.