Make search.findElementReferences.result.element field optional.

Make search.findElementReferences.result.element field optional.

This resolves an inconsistency between the spec and the analysis
server behavior: the server doesn't populete the "element" field if no
element could be found.

R=brianwilkerson@google.com

Committed: https://code.google.com/p/dart/source/detail?r=39388

[Paul Berry, 2014-08-19 20:33:37]

Note: I'm not sure whether or not this is what we want in the long run--maybe it
would be better for search.findElementReferences to generate an error if no
element can be found?

But in any case, I need the analysis server behavior and the API spec to agree
in the short term so that I can continue porting the analysis server code over
to using the generated classes.

[Brian Wilkerson, 2014-08-19 21:52:07]

LGTM, but we should revisit the larger question later

[Paul Berry, 2014-08-19 21:56:35]

Committed patchset #1 manually as 39388 (presubmit successful).