Add PrefixElement linkage to analyzer scope lookups

lib/src/markdown_processor.dart

@@ -309,6 +309,12 @@ MatchingLinkResult _getMatchingLinkElementCommentReferable(
   var lookupResult =
       warnable.referenceBy(commentReference.referenceBy, filter: filter);
 
+  // TODO(jcollins-g): Referring to packages or other non-[ModelElement]s
+  // might be needed here.  Determine if that's the case.
+  if (!(lookupResult is ModelElement)) {
+    lookupResult = null;
+  }
+
   // TODO(jcollins-g): Consider prioritizing analyzer resolution before custom.
   return MatchingLinkResult(lookupResult);
 }

lib/src/model/comment_referable.dart

@@ -79,17 +79,12 @@ mixin CommentReferable implements Nameable {
   /// Looks up references by [scope], skipping over results that do not match
   /// the given filter.
   ///
-  /// Override if [Scope.lookup] may return a [PrefixElement] or other elements
-  /// not corresponding to a [CommentReferable], but you still want to have
-  /// an implementation of [scope].
+  /// Override if [Scope.lookup] may return elements not corresponding to a
+  /// [CommentReferable], but you still want to have an implementation of
+  /// [scope].
   CommentReferable lookupViaScope(ReferenceChildrenLookup referenceLookup,
       bool Function(CommentReferable) filter) {
     var resultElement = scope.lookupPreferGetter(referenceLookup.lookup);
-    if (resultElement is PrefixElement) {
-      assert(false,
-          'PrefixElement detected, override [lookupViaScope] in subclass');
-      return null;
-    }
     if (resultElement == null) return null;
     var result = ModelElement.fromElement(resultElement, packageGraph);
     if (result is Accessor) {

lib/src/model/model_element.dart

@@ -22,6 +22,7 @@ import 'package:dartdoc/src/model/comment_referable.dart';
 import 'package:dartdoc/src/model/feature.dart';
 import 'package:dartdoc/src/model/feature_set.dart';
 import 'package:dartdoc/src/model/model.dart';
+import 'package:dartdoc/src/model/prefix.dart';
 import 'package:dartdoc/src/model_utils.dart' as utils;
 import 'package:dartdoc/src/render/model_element_renderer.dart';
 import 'package:dartdoc/src/render/parameter_renderer.dart';
@@ -286,6 +287,9 @@ abstract class ModelElement extends Canonicalization
     if (e is LibraryElement) {
       return Library(e, packageGraph);
     }
+    if (e is PrefixElement) {
+      return Prefix(e, library, packageGraph);
+    }
     if (e is ClassElement) {
       if (e.isMixin) {
         return Mixin(e, library, packageGraph);

lib/src/model/parameter.dart

@@ -31,7 +31,7 @@ class Parameter extends ModelElement implements EnclosedElement {
 
   @override
   String get filePath {
-    throw StateError('filePath not implemented for parameters');
+    throw UnimplementedError('Parameters have no generated files in dartdoc');
   }
 
   @override

lib/src/model/prefix.dart

@@ -0,0 +1,49 @@
+// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer/dart/element/scope.dart';
+import 'package:dartdoc/src/model/comment_referable.dart';
+import 'package:dartdoc/src/model/library.dart';
+
+import '../../dartdoc.dart';
+
+/// Represents a [PrefixElement] for dartdoc.
+///
+/// Like [Parameter], it doesn't have doc pages, but participates in lookups.
+class Prefix extends ModelElement implements EnclosedElement {
+  /// [library] is the library the prefix is defined in, not the [Library]
+  /// referred to by the [PrefixElement].
+  Prefix(PrefixElement element, Library library, PackageGraph packageGraph)
+      : super(element, library, packageGraph);
+
+  @override
+  // TODO(jcollins-g): consider allowing bare prefixes to link to a library doc?
+  bool get isCanonical => false;
+
+  @override
+  Scope get scope => element.scope;
+
+  @override
+  PrefixElement get element => super.element;
+
+  @override
+  ModelElement get enclosingElement => library;
+
+  @override
+  String get filePath =>
+      throw UnimplementedError('prefixes have no generated files in dartdoc');
+
+  @override
+  String get href => null;
+
+  @override
+  String get kind => 'prefix';
+
+  @override
+  Map<String, CommentReferable> get referenceChildren => {};
+
+  @override
+  Iterable<CommentReferable> get referenceParents => [library];
+}

test/end2end/model_test.dart

@@ -2264,8 +2264,7 @@ void main() {
           equals(MatchingLinkResult(incorrectDocReferenceFromEx)));
 
       // A prefixed constant in another library.
-      // TODO(jcollins-g): prefixed namespace lookups are not yet implemented with new lookup code.
-      expect(originalLookup(doAwesomeStuff, 'css.theOnlyThingInTheLibrary'),
+      expect(bothLookup(doAwesomeStuff, 'css.theOnlyThingInTheLibrary'),
           equals(MatchingLinkResult(theOnlyThingInTheLibrary)));
 
       // A name that exists in this package but is not imported.