Add a unit index to prelinked type references in summaries.

Add a unit index to prelinked type references in summaries.

In order to avoid having to reconstitute the entire element model when
deserializing a library summary, we'll need to be able to create element
handles for any elements referenced by the library.  Since element
handles operate using a "location", and the "location" records
information about the compilation unit containing the element, this
means that the library summary needs to track which unit each referenced
element lives in (even though this information would otherwise not be
semantically relevant).

R=scheglov@google.com

Committed: https://github.com/dart-lang/sdk/commit/84c3cdb0487b2a4d546cf08a59607969d98fa533

[Paul Berry, 2015-11-20 21:19:30]

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

[scheglov, 2015-11-20 22:00:17]

LGTM

[Paul Berry, 2015-11-20 22:10:46]

Committed patchset #2 (id:20001) manually as
84c3cdb0487b2a4d546cf08a59607969d98fa533 (presubmit successful).