Fix issue with extension members and unfound doc comment references

lib/src/model/comment_referable.dart

@@ -104,10 +104,12 @@ mixin CommentReferable implements Nameable {
       if (resultElement == null) return null;
     }
 
-    if (this case ModelElement(:var modelNode?) when resultElement == null) {
-      var references = modelNode.commentData?.references;
-      if (references != null) {
-        resultElement = references[referenceLookup.lookup]?.element;
+    if (resultElement == null) {
+      if (this case ModelElement(:var modelNode?)) {
+        var references = modelNode.commentData?.references;
+        if (references != null) {
+          resultElement = references[referenceLookup.lookup]?.element;
+        }
       }
     }
 

lib/src/model/container_member.dart

@@ -61,6 +61,9 @@ mixin ContainerMember on ModelElement {
       // Until then, just pretend we're handling this correctly.
       [
         (documentationFrom.first as ModelElement).definingLibrary,
-        (packageGraph.findCanonicalModelElementFor(this) ?? this).library,
-      ];
+        if (this case Field(:var getter, :var setter))
+          packageGraph.findCanonicalModelElementFor(getter ?? setter)?.library
+        else
+          (packageGraph.findCanonicalModelElementFor(this) ?? this).library,
+      ].nonNulls.toList();
 }

lib/src/model/model_element.dart

@@ -234,7 +234,6 @@ abstract class ModelElement
       originalMember: originalMember,
     );
 
-    if (enclosingContainer != null) assert(newModelElement is Inheritable);
     _cacheNewModelElement(e, newModelElement, library,
         enclosingContainer: enclosingContainer);
 
@@ -324,11 +323,10 @@ abstract class ModelElement
     if (e.enclosingElement is ExtensionElement ||
         e.enclosingElement is InterfaceElement ||
         e is MultiplyInheritedExecutableElement) {
-      if (enclosingContainer == null) {
+      if (enclosingContainer == null || enclosingContainer is Extension) {
         return ContainerAccessor(e, library, packageGraph);
       }
 
-      assert(e.enclosingElement is! ExtensionElement);
       return ContainerAccessor.inherited(
           e, library, packageGraph, enclosingContainer,
           originalMember: originalMember as ExecutableMember?);

lib/src/model/package_graph.dart

@@ -769,7 +769,7 @@ class PackageGraph with CommentReferable, Nameable {
       // TODO(keertip): Find a better way to exclude members of extensions
       // when libraries are specified using the "--include" flag.
       if (lib != null && lib.isDocumented) {
-        return getModelFor(element, lib);
+        return getModelFor(element, lib, enclosingContainer: preferredClass);
       }
     }
     // TODO(jcollins-g): The data structures should be changed to eliminate

test/extensions_test.dart

@@ -37,6 +37,22 @@ var f() {}
     );
   }
 
+  void test_referenceToMissingElement_onExtensionMember() async {
+    var library = await bootPackageWithLibrary('''
+extension E on int {
+  /// Text [NotFound] text.
+  int get f => 7;
+}
+''');
+
+    // We are primarily testing that dartdoc does not crash when trying to
+    // resolve an unknown reference, from the position of an extension member.
+    expect(
+      library.extensions.first.instanceFields.first.documentationAsHtml,
+      contains('<p>Text <code>NotFound</code> text.</p>'),
+    );
+  }
+
   void test_referenceToExtensionMethod() async {
     var library = await bootPackageWithLibrary('''
 extension Ex on int {