Better structure for the is-library-detection code. (#1423)

lib/src/dartdoc/dartdoc_index.dart

@@ -6,6 +6,8 @@ import 'dart:convert';
 
 import 'package:json_annotation/json_annotation.dart';
 
+import 'dartdoc_internals.dart';
+
 part 'dartdoc_index.g.dart';
 
 /// The parsed content of the `index.json` generated by dartdoc.
@@ -71,9 +73,7 @@ class DartdocIndexEntry {
   /// Weather the entry is a top-level library:
   /// - pre-8.3.0 the file ended with `-library.html`
   /// - with 8.3.0 the reference is a top-level directory with no slash in it
-  late final isLibrary = href != null &&
-      href!.isNotEmpty &&
-      (href!.endsWith('-library.html') || !href!.contains('/'));
+  late final isLibrary = isHrefALibrary(href);
 
   /// Whether the entry is a class declaration.
   bool get isClass => href != null && href!.endsWith('-class.html');

lib/src/dartdoc/dartdoc_internals.dart

@@ -0,0 +1,22 @@
+// Copyright (c) 2024, 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.
+
+/// Infers whether a dartdoc href is a link to a library page.
+/// - pre-8.3.0 the file ended with `-library.html`
+/// - with 8.3.0 the reference is a top-level directory with no slash in it
+bool isHrefALibrary(String? href) {
+  if (href == null || href.isEmpty) {
+    return false;
+  }
+  // libraries before 8.3
+  if (href.endsWith('-library.html')) {
+    return true;
+  }
+  // libraries after 8.3 do not have the .html suffix in their href
+  if (href.endsWith('.html')) {
+    return false;
+  }
+  // libraries have no slash in their hrefs
+  return !href.contains('/');
+}

lib/src/dartdoc/pub_dartdoc_data.dart

@@ -4,6 +4,8 @@
 
 import 'package:json_annotation/json_annotation.dart';
 
+import 'dartdoc_internals.dart';
+
 part 'pub_dartdoc_data.g.dart';
 
 @JsonSerializable()
@@ -55,9 +57,7 @@ class ApiElement {
   /// Weather the entry is a top-level library:
   /// - pre-8.3.0 the file ended with `-library.html`
   /// - with 8.3.0 the reference is a top-level directory with no slash in it
-  late final isLibrary = href != null &&
-      href!.isNotEmpty &&
-      (href!.endsWith('-library.html') || !href!.contains('/'));
+  late final isLibrary = isHrefALibrary(href);
 
   /// Whether the entry is a class declaration.
   late final isClass =