Fix crash on missing libraries.json

[annagrin]

Emit a warning instead of crashing on missing libraries spec (libraries.json), add tests.

[nshahan]

LGTM, with a few questions.

Now what happens after the warning message is logged? Does the program exit? Can it actually continue without the libraries.json file?

What is the libraries.json file used for? My only familiarity with it is that it will be used by the CFE when compiling the SDK. Since we precompile the SDK .dill files and vend them with the SDK I wouldn't expect a need to read this file.

[annagrin]

Now what happens after the warning message is logged? Does the program exit? Can it actually continue without the libraries.json file?

dwds continues to work but does not support lookupResolvedPackageUris and lookupPackageUris VM API (returns empty results). Those are currently only used in dart-code, and are new, so we are not breaking anything by not being able to find the file. Currently many tools that use dwds do not have access to libraries.json, so this fix defaults to existing behavior in those cases instead of introducing a new crash. The next steps would be to provide libraries.json in all uses of dwds.

I'll add that information to the warning reported.

What is the libraries.json file used for? My only familiarity with it is that it will be used by the CFE when compiling the SDK. Since we precompile the SDK .dill files and vend them with the SDK I wouldn't expect a need to read this file.

It is used to resolve uris - see implementations of lookupResolvedPackageUris and lookupPackageUris in chrome_proxy_service.dart:

webdev/dwds/lib/src/services/chrome_proxy_service.dart

Line 659 in 6e3200e

Future<UriList> lookupResolvedPackageUris(

and its underlying implementation in dart_uri.dart:

webdev/dwds/lib/src/utilities/dart_uri.dart

Line 218 in 07d1034

var libSpec = _librariesSpec?.libraryInfoFor(uri.path);

[annagrin]

From offline conversation - we need to revisit usage of libraries.json in dwds. Can dwds resolve dart: uris in a different way? Examples: add an API to the incremental ddc compiler, save in sdk metadata?