Generate the diagnostic pages from data files (#7061)

- Updates `dart run dash_site generate-diagnostics` to generate a
`src/data/diagnostics.json` file from data in the SDK. This file is not
meant to be read or manually edited. It's mostly just a mechanism to
cache the results calculated from the SDK data.
- Stops manually generating the individual diagnostic doc pages found at
`/tools/diagnostics/<diagnostic_name>`. Instead use the data from the
intermediate `src/data/diagnostics.json` file to generate each page from
code. The content of the generated files remains the same with this PR.

These changes reduce site cloning and full build time, make updates
easier to implement and review, and help avoid confusion about those
files being the source of the docs.

Author: parlough

site/lib/src/components/common/tags.dart

@@ -23,26 +23,46 @@ class Tags extends StatelessComponent {
 ///
 /// Generally displayed within a [Tags] component.
 class Tag extends StatelessComponent {
-  const Tag(this.content, {this.icon, this.title, this.label, this.color});
+  const Tag(
+    this.content, {
+    this.icon,
+    this.title,
+    this.label,
+    this.color,
+    this.link,
+  });
 
   final String content;
   final String? icon;
   final String? title;
   final String? label;
   final String? color;
+  final String? link;
 
   @override
   Component build(BuildContext context) {
+    final children = <Component>[
+      if (icon case final iconId?) MaterialIcon(iconId),
+      span([.text(content)]),
+    ];
+    final attributes = {
+      'title': ?title,
+      'aria-label': ?(label ?? title),
+    };
+
+    if (link case final link?) {
+      return a(
+        href: link,
+        classes: 'tag-label',
+        attributes: attributes,
+        children,
+      );
+    }
+
     return div(
       classes: 'tag-label',
-      attributes: {
-        'title': ?title,
-        'aria-label': ?(label ?? title),
-      },
-      [
-        if (icon case final iconId?) MaterialIcon(iconId),
-        span([.text(content)]),
-      ],
+      attributes: attributes,
+      children,
     );
   }
 }

site/lib/src/models/diagnostic_model.dart

@@ -0,0 +1,44 @@
+// Copyright (c) 2025, 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 'dart:convert' show jsonDecode;
+import 'dart:io';
+
+import 'package:path/path.dart' as p;
+
+import '../util.dart';
+
+extension type DiagnosticInfo.fromJson(Map<String, Object?> info) {
+  String get id => info['id'] as String;
+  String get description => info['description'] as String;
+  String? get documentation => info['documentation'] as String?;
+  bool get hasDocumentation => info['hasDocumentation'] as bool? ?? false;
+  bool get fromLint => info['fromLint'] as bool? ?? false;
+  List<String> get previousNames =>
+      (info['previousNames'] as List<Object?>).cast<String>();
+}
+
+/// Reads and parses information about diagnostics from
+/// the `src/data/diagnostics.json` file.
+List<DiagnosticInfo> readAndLoadDiagnostics() {
+  if (_loadedDiagnostics case final alreadyLoadedDiagnostics?) {
+    return alreadyLoadedDiagnostics;
+  }
+
+  final diagnosticsFile = File(
+    p.join(siteSrcDirectoryPath, 'data', 'diagnostics.json'),
+  );
+  final rawDiagnosticInfo =
+      jsonDecode(diagnosticsFile.readAsStringSync()) as List<Object?>;
+
+  final diagnostics = rawDiagnosticInfo
+      .cast<Map<String, Object?>>()
+      .map(DiagnosticInfo.fromJson)
+      .toList(growable: false);
+
+  return _loadedDiagnostics = diagnostics;
+}
+
+/// A cache of the loaded and parsed diagnostic info.
+List<DiagnosticInfo>? _loadedDiagnostics;