Add rudimentary freshness tool (#5959)

Added for determining freshness. Nothing happens in CI, this is just for
running locally.

Used like:

```
./dash_site freshness
```

with output like:

```
====== Files with missing lastVerified dates ======

❓   missing-date   web/libraries.md
❓   missing-date   web/wasm.md

====== Files with stale lastVerified dates ======

🟥    2021-05-13    resources/google-apis.md
🟧    2022-11-21    resources/coming-from/js-to-dart.md
🟧    2022-12-13    resources/coming-from/swift-to-dart.md
🟨    2023-02-23    guides/environment-declarations.md

```

You can optionally change the cutoff to include a file as stale with the
`--cutoff` option which accepts an integer amount of days. It currently
defaults to 270 days.

The staleness is determined by the `lastVerified` option in each page's
YAML frontmatter. `lastVerified` indicates when the page was last
verified as up to date as of:

```yaml
---
title: Document title
lastVerified: 2024-04-21
---
```

If a file shouldn't be checked for freshness, you can specify
`skipFreshness: true` in the frontmatter:

```yaml
---
title: Document title
skipFreshness: true
---
```

Author: parlough

src/content/404.html

@@ -4,6 +4,7 @@
 description: "dart.dev's 404 page."
 sitemap: false
 permalink: /404.html
+skipFreshness: true
 ---
 
 <div class="text-center">

src/content/brand.md

@@ -2,6 +2,7 @@
 title: Dart brand guidelines
 description: >-
   The guidelines governing the usage of the Dart trademarks and assets.
+skipFreshness: true
 ---
 
 The "Dart" name and logo are trademarks owned by Google.

src/content/community/code-of-conduct.md

@@ -2,6 +2,7 @@
 title: Code of conduct
 description: Be excellent to each other.
 toc: false
+skipFreshness: true
 ---
 
 We expect Dart community members to act professionally and respectfully, and

src/content/guides/environment-declarations.md

@@ -3,6 +3,7 @@ title: Configuring apps with compilation environment declarations
 description: >-
   Learn about using compilation environment declarations 
   to customize application behavior.
+lastVerified: 2023-02-23
 ---
 
 You can specify compilation environment declarations

src/content/guides/language/evolution.md

@@ -2,6 +2,7 @@
 title: Dart language evolution
 short-title: Language evolution
 description: Notable changes and additions to the Dart programming language.
+lastVerified: 2024-05-14
 ---
 
 This page lists notable changes and additions to the

src/content/guides/language/language-tour.md

@@ -6,6 +6,7 @@ js: [{url: '/assets/js/language-tour-redirector.js'}]
 sitemap: false
 canonical: https://dart.dev/language
 noindex: true
+skipFreshness: true
 ---
 
 :::warning

src/content/guides/libraries/library-tour.md

@@ -6,6 +6,7 @@ js: [{url: '/assets/js/library-tour-redirector.js'}]
 sitemap: false
 canonical: https://dart.dev/libraries
 noindex: true
+skipFreshness: true
 ---
 
 :::warning

src/content/language/concurrency.md

@@ -3,6 +3,7 @@ title: Concurrency in Dart
 description: >-
   Use isolates to enable parallel code execution on multiple processor cores.
 short-title: Concurrency
+lastVerified: 2023-12-14
 prevpage:
   url: /language/modifier-reference
   title: Class modifiers reference

src/content/language/isolates.md

@@ -2,6 +2,7 @@
 title: Isolates
 description: Information on writing isolates in Dart.
 short-title: Isolates
+lastVerified: 2024-01-04
 prevpage:
   url: /language/async
   title: Asynchronous support

src/content/map.html

@@ -2,6 +2,7 @@
 title: Document index
 description: Human-readable sitemap of all pages on dart.dev.
 toc: false
+skipFreshness: true
 ---
 
 {% assign pages = collections.all | sort: 'data.title' %}

src/content/resources/breaking-changes.md

@@ -1,6 +1,7 @@
 ---
 title: Breaking changes and deprecations
 description: A list of breaking changes by release in Dart.
+lastVerified: 2024-05-14
 ---
 
 {% assign versioned = '<span class="tag-label language-versioned-tag">Language versioned</span>' %}

src/content/resources/coming-from/js-to-dart.md

@@ -2,6 +2,7 @@
 title: Learning Dart as a JavaScript developer
 description: Leverage your JavaScript knowledge when learning Dart.
 body_class: highlight-languages
+lastVerified: 2022-11-21
 ---
 
 This guide aims to leverage your JavaScript programming knowledge

src/content/resources/coming-from/swift-to-dart.md

@@ -2,6 +2,7 @@
 title: Learning Dart as a Swift developer
 description: Leverage your Swift knowledge when learning Dart.
 body_class: highlight-languages
+lastVerified: 2022-12-13
 ---
 
 This guide aims to leverage your Swift programming

src/content/resources/google-apis.md

@@ -2,6 +2,7 @@
 title: Using Google APIs
 short-title: Google APIs
 description: Your Dart apps can use Firebase and Google client APIs.
+lastVerified: 2021-05-13
 ---
 
 This page points to resources to help you use

src/content/robots.liquid

@@ -1,6 +1,7 @@
 ---
 layout: none
 permalink: robots.txt
+skipFreshness: true
 ---
 
 {%- if isProduction == true -%}

src/content/search-all.html

@@ -4,6 +4,7 @@
   Search dart.dev, api.dart.dev, the old www.dartlang.org site, 
   flutter.dev, and more.
 toc: false
+skipFreshness: true
 ---
 
 <p>

src/content/search.html

@@ -3,6 +3,7 @@
 short-title: Search
 description: Search for documentation on dart.dev.
 toc: false
+skipFreshness: true
 ---
 
 Want results from additional Dart-related sites, like api.dart.dev and flutter.dev?

src/content/security.md

@@ -2,6 +2,7 @@
 title: Security
 description: >-
   An overview of the Dart team's philosophy and processes for security.
+skipFreshness: true
 ---
 
 The Dart team takes the security of Dart and the applications

src/content/sitemap.liquid

@@ -2,6 +2,7 @@
 layout: none
 sitemap: false
 permalink: sitemap.xml
+skipFreshness: true
 ---
 <?xml version="1.0" encoding="UTF-8"?>
 <urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">

src/content/terms.md

@@ -2,6 +2,7 @@
 title: Terms of use
 description: The terms of use for dart.dev.
 toc: false
+skipFreshness: true
 ---
 
 The Dart website (the "Website") is hosted by Google.

src/content/tools/diagnostic-messages.md

@@ -2,6 +2,7 @@
 title: Diagnostic messages
 description: Details for diagnostics produced by the Dart analyzer.
 body_class: highlight-diagnostics
+skipFreshness: true
 ---
 {%- comment %}
 WARNING: Do NOT EDIT this file directly. It is autogenerated by the script in

src/content/tools/linter-rules/individual-rules.md

@@ -10,6 +10,7 @@ eleventyComputed:
   permalink: "/tools/linter-rules/{{lint.name}}.html"
   title: "{{ lint.name }}"
   description: "Learn more about the {{ lint.name }} linter rule."
+skipFreshness: true
 ---
 
 {{lint.description}}

tool/dart_site/lib/dart_site.dart

@@ -11,6 +11,7 @@ import 'src/commands/check_link_references.dart';
 import 'src/commands/check_links.dart';
 import 'src/commands/check_site_variable.dart';
 import 'src/commands/format_dart.dart';
+import 'src/commands/freshness.dart';
 import 'src/commands/generate_effective_dart_toc.dart';
 import 'src/commands/refresh_excerpts.dart';
 import 'src/commands/serve.dart';
@@ -32,6 +33,7 @@ final class DartSiteCommandRunner extends CommandRunner<int> {
     addCommand(VerifyFirebaseJsonCommand());
     addCommand(RefreshExcerptsCommand());
     addCommand(FormatDartCommand());
+    addCommand(FreshnessCommand());
     addCommand(GenerateEffectiveDartToc());
     addCommand(AnalyzeDartCommand());
     addCommand(TestDartCommand());