Allow sending duplicate notifications.

Allow sending duplicate notifications.

This will "unfix" dartbug.com/22210 , but at the same time it will
remove most of of expensive AnalysisContext calculations introduced
in https://code.google.com/p/dart/source/detail?r=44305

It reduces "notices" tag percentage from 15 to a little above 3.

Also, we don't produce errors for external sources, which was the
main problem in dartbug.com/22210.

There is also the same expensive computation in execution domain.
I will address it in a separate CL.


R=brianwilkerson@google.com, paulberry@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=44330

[Brian Wilkerson, 2015-03-07 00:48:16]

LGTM

[scheglov, 2015-03-09 16:57:35]

Committed patchset #1 (id:1) manually as 44330 (presubmit successful).