Fix regression in import fixups introduced by .packages support.

Fix regression in import fixups introduced by .packages support.

With the introduction of support for ".packages" files, analysis
server now uses a slightly different code path for translating the
Source objects from imported package files into "package:" URI's.
Previously it used PackageUriResolver.resolveAbsolute(); now it uses
ResourceUriResolver.resolveAbsolute(), and then translates the
resulting file: URI to a package: URI uring the .packages file.

The old code path used Source.fullName (the file path) to locate the
file in the filesystem; the new code path uses Source.uri.  This
causes problems because when figuring out the correct import statement
to insert, the analysis server creates a bogus Source object whose uri
field is null.  As a result, import fixups were getting relative paths
like "../../../../.pub-cache/..." rather than package imports, which
is clearly wrong.

As a short term non-invasive fix, we set Source.uri in the bogus
Source object, so that the new codepath is able to find the path of
the imported file.

R=brianwilkerson@google.com

Committed: https://github.com/dart-lang/sdk/commit/f3f52d4b59081e2f294126ac7f69f902968692d7

[Paul Berry, 2015-08-10 18:22:48]

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

[Brian Wilkerson, 2015-08-10 18:32:12]

LGTM

[Paul Berry, 2015-08-10 19:14:31]

Committed patchset #2 (id:20001) manually as
f3f52d4b59081e2f294126ac7f69f902968692d7 (presubmit successful).