Warn when the defining library cannot be found (#2319)

Author: srawlins

lib/src/model/model_element.dart

@@ -658,8 +658,13 @@ abstract class ModelElement extends Canonicalization
         documentationFrom.map((e) => e.documentationLocal).join('<p>'));
   }
 
-  Library get definingLibrary =>
-      packageGraph.findButDoNotCreateLibraryFor(element);
+  Library get definingLibrary {
+    var library = packageGraph.findButDoNotCreateLibraryFor(element);
+    if (library == null) {
+      warn(PackageWarning.noDefiningLibraryFound);
+    }
+    return library;
+  }
 
   Library _canonicalLibrary;
 
@@ -700,6 +705,9 @@ abstract class ModelElement extends Canonicalization
   }
 
   Library _searchForCanonicalLibrary() {
+    if (definingLibrary == null) {
+      return null;
+    }
     var thisAndExported = definingLibrary.exportedInLibraries;
 
     if (thisAndExported == null) {
@@ -1023,7 +1031,7 @@ abstract class ModelElement extends Canonicalization
   PackageGraph get packageGraph => _packageGraph;
 
   @override
-  Package get package => library.package;
+  Package get package => library?.package;
 
   bool get isPublicAndPackageDocumented =>
       isPublic && library.packageGraph.packageDocumentedFor(this);

lib/src/model/package_graph.dart

@@ -347,6 +347,11 @@ class PackageGraph {
         warningMessage =
             'ambiguous reexport of ${warnableName}, canonicalization candidates: ${message}';
         break;
+      case PackageWarning.noDefiningLibraryFound:
+        warningMessage =
+            'could not find the defining library for ${warnableName}; the '
+            'library may be imported or exported with a non-standard URI';
+        break;
       case PackageWarning.noLibraryLevelDocs:
         warningMessage =
             '${warnable.fullyQualifiedName} has no library level documentation comments';

lib/src/warnings.dart

@@ -135,11 +135,20 @@ final Map<PackageWarning, PackageWarningDefinition> packageWarningDefinitions =
   PackageWarning.ignoredCanonicalFor: PackageWarningDefinition(
       PackageWarning.ignoredCanonicalFor,
       'ignored-canonical-for',
-      'A @canonicalFor tag refers to a library which this symbol can not be canonical for'),
+      'A @canonicalFor tag refers to a library which this symbol can not be '
+          'canonical for'),
   PackageWarning.noCanonicalFound: PackageWarningDefinition(
       PackageWarning.noCanonicalFound,
       'no-canonical-found',
-      'A symbol is part of the public interface for this package, but no library documented with this package documents it so dartdoc can not link to it'),
+      'A symbol is part of the public interface for this package, but no '
+          'library documented with this package documents it so dartdoc can '
+          'not link to it'),
+  PackageWarning.noDefiningLibraryFound: PackageWarningDefinition(
+      PackageWarning.noDefiningLibraryFound,
+      'no-defining-library-found',
+      'The defining library for an element could not be found; the library may '
+          'be imported or exported with a non-standard URI',
+      defaultWarningMode: PackageWarningMode.error),
   PackageWarning.notImplemented: PackageWarningDefinition(
       PackageWarning.notImplemented,
       'not-implemented',
@@ -245,6 +254,7 @@ enum PackageWarning {
   ambiguousReexport,
   ignoredCanonicalFor,
   noCanonicalFound,
+  noDefiningLibraryFound,
   notImplemented,
   noLibraryLevelDocs,
   packageOrderGivesMissingPackageName,
@@ -441,9 +451,7 @@ class PackageWarningCounter {
     PackageWarningOptionContext config =
         element?.config ?? packageGraph.defaultPackage.config;
     var warningMode = config.packageWarningOptions.getMode(kind);
-    if (!config.allowNonLocalWarnings &&
-        element != null &&
-        !element.package.isLocal) {
+    if (!config.allowNonLocalWarnings && !(element?.package?.isLocal ?? true)) {
       warningMode = PackageWarningMode.ignore;
     }
     if (warningMode == PackageWarningMode.warn) {