Packages/libraries referrred via prefixed imports are sometimes missing from package graph

[jcollins-g]

Dartdoc is supposed to add all packages accessible via the import/export graph to the PackageGraph at initialization, but packages containing libraries only referred to via import prefixes seem to be missing from the data structure. This will result in an exception in the link validator if someone is referring to a member of that library via the import prefix.

Fuchsia noticed this error, but used --no-enhanced-reference-lookup to work around it. This probably worked because the old reference lookup system had very buggy/incomplete support for prefixes, and so it seems likely that we just didn't resolve the links that we're now trying to verify.

dartdoc 3.0.0 (/usr/local/google/home/theosiu/dartdoc/bin/dartdoc.dart) failed: NoSuchMethodError: The getter 'library' was called on null.
Receiver: null
Tried calling: library
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:63:5)
#1      PackageGraph.findButDoNotCreateLibraryFor (package:dartdoc/src/model/package_graph.dart:888:27)
#2      new ModelElement.fromElement (package:dartdoc/src/model/model_element.dart:131:17)
#3      Prefix.associatedLibrary (package:dartdoc/src/model/prefix.dart:30:43)
#4      Prefix.canonicalModelElement (package:dartdoc/src/model/prefix.dart:37:40)
#5      Prefix.href (package:dartdoc/src/model/prefix.dart:54:7)
#6      PackageGraph.allHrefs (package:dartdoc/src/model/package_graph.dart:581:33)
#7      Dartdoc._validateLinks (package:dartdoc/dartdoc.dart:485:27)
#8      Dartdoc.generateDocsBase (package:dartdoc/dartdoc.dart:217:7)
<asynchronous suspension>
#9      Dartdoc.generateDocs (package:dartdoc/dartdoc.dart:249:28)
<asynchronous suspension>
#10     Dartdoc.executeGuarded.<anonymous closure> (package:dartdoc/dartdoc.dart:512:9)
<asynchronous suspension>

[jcollins-g]

Some of this may be actually be a side effect of import/export graph problems, see #2789. Investigation is continuing.

[jcollins-g]

I am not certain that this is a P1 issue anymore -- prefixed import support has improved since this was filed. I think we need to reinvestigate whether this is still a problem.