Change mixin implements and remove unused Extendable.

lib/src/model/documentation_comment.dart

@@ -35,7 +35,7 @@ final _htmlInjectRegExp = RegExp(r'<dartdoc-html>([a-f0-9]+)</dartdoc-html>');
 /// [_processCommentWithoutTools] and [processComment] are the primary
 /// entrypoints.
 mixin DocumentationComment
-    on Canonicalization, Documentable, Warnable, Locatable, SourceCode {
+    implements Canonicalization, Documentable, Warnable, Locatable, SourceCode {
   @override
   Element get element;
 

lib/src/model/has_no_page.dart

@@ -5,7 +5,7 @@
 import 'package:dartdoc/src/model/model.dart';
 
 /// An element which has no page documenting itself.
-mixin HasNoPage on ModelElement {
+mixin HasNoPage implements ModelElement {
   @override
   String get filePath =>
       throw UnimplementedError('This element has no generated page');

lib/src/model/inheriting_container.dart

@@ -18,7 +18,7 @@ import 'package:meta/meta.dart';
 ///
 /// Note that [Constructor]s are not considered to be modifiers so a
 /// [hasModifiers] override is not necessary for this mixin.
-mixin Constructable on InheritingContainer {
+mixin Constructable implements InheritingContainer {
   late final List<Constructor> constructors = element.constructors
       .map((e) => getModelFor(e, library) as Constructor)
       .toList(growable: false);

lib/src/model/model.dart

@@ -17,7 +17,6 @@ export 'documentation_comment.dart';
 export 'dynamic.dart';
 export 'enclosed_element.dart';
 export 'enum.dart';
-export 'extendable.dart';
 export 'extension.dart';
 export 'extension_type.dart';
 export 'field.dart';