Fixed crash when compiling without linking.

Fixed crash when compiling without linking.

Since offsets and uris was added to the dill file, compiling to dill
without linking hasn't worked because of external libraries.

The external libraries doesn't add to the line starts table and thus
the uris from libraries, classes and so on isn't recorded.

When trying to write an unrecorded uri to the dill file it fails.

The simple solution applied here is to fall-back to the empty
string when the uri hasn't been recorded.

BUG=
R=asgerf@google.com

Committed: https://github.com/dart-lang/kernel/commit/b044c217f5f96e30828f9e6adf6cd9e58364021d

[jensj, 2016-11-08 13:37:11]

jensj@google.com changed reviewers:
+ asgerf@google.com

[asgerf, 2016-11-08 13:44:38]

CL description:
The "dummy loads" are called external libraries. There is no reason to call them
dummy loads.

In most cases I would also try to avoid using the first person in CL
descriptions.

Otherwise LGTM.

[jensj, 2016-11-08 13:53:31]

Description was changed from

==========
Fixed crash when compiling without linking.

Since I added offsets and uris to the dill file,
it seem that compiling to dill without linking hasn't worked
because it "dummy loads" libraries referenced.
These dummy-loads doesn't add to the line starts table and
thus the uri the libraries, classes and so on comes from
isn't recorded. When trying to write the uri it hasn't been
indexed either, and writing then fails after a failed lookup
of the uri.
The simple solution applied here is to fall-back to the empty
string when the uri is unknown.

BUG=
==========

to

==========
Fixed crash when compiling without linking.

Since offsets and uris was added to the dill file, compiling to dill
without linking hasn't worked because of external libraries.

The external libraries doesn't add to the line starts table and thus
the uris from libraries, classes and so on isn't recorded.

When trying to write an unrecorded uri to the dill file it fails.

The simple solution applied here is to fall-back to the empty
string when the uri hasn't been recorded.

BUG=
==========

[jensj, 2016-11-08 13:54:20]

Description was changed from

==========
Fixed crash when compiling without linking.

Since offsets and uris was added to the dill file, compiling to dill
without linking hasn't worked because of external libraries.

The external libraries doesn't add to the line starts table and thus
the uris from libraries, classes and so on isn't recorded.

When trying to write an unrecorded uri to the dill file it fails.

The simple solution applied here is to fall-back to the empty
string when the uri hasn't been recorded.

BUG=
==========

to

==========
Fixed crash when compiling without linking.

Since offsets and uris was added to the dill file, compiling to dill
without linking hasn't worked because of external libraries.

The external libraries doesn't add to the line starts table and thus
the uris from libraries, classes and so on isn't recorded.

When trying to write an unrecorded uri to the dill file it fails.

The simple solution applied here is to fall-back to the empty
string when the uri hasn't been recorded.

BUG=
R=asgerf@google.com

Committed:
https://github.com/dart-lang/kernel/commit/b044c217f5f96e30828f9e6adf6cd9e583...
==========

[jensj, 2016-11-08 13:54:22]

Committed patchset #1 (id:1) manually as
b044c217f5f96e30828f9e6adf6cd9e58364021d (presubmit successful).