Move sound problems doc out of guides/ directory (#6264)

- Move the sound-problems doc to the deprecated directory, as most of it
is duplicated with diagnostic message docs. See
#6265 for follow up work.
- Delete the `/guides/language` index as it's not linked to from
anywhere and mostly duplicates introductions from other pages.

Contributes to #5767
Contributes to #6265

firebase.json

@@ -210,11 +210,12 @@
       { "source": "/googleapis", "destination": "https://github.com/dart-lang/googleapis", "type": 301 },
       { "source": "/guides/get-started", "destination": "/overview", "type": 301 },
       { "source": "/guides/google-apis", "destination": "/resources/google-apis", "type": 301 },
+      { "source": "/guides/language", "destination": "/language", "type": 301 },
       { "source": "/guides/language/analysis-options", "destination": "/tools/analysis", "type": 301 },
       { "source": "/guides/language/cheatsheet", "destination": "/language", "type": 301 },
       { "source": "/guides/language/coming-from/js-to-dart", "destination": "/resources/coming-from/js-to-dart", "type": 301 },
       { "source": "/guides/language/coming-from/swift-to-dart", "destination": "/resources/coming-from/swift-to-dart", "type": 301 },
-      { "source": "/guides/language/common-prob", "destination": "/guides/language/sound-problems", "type": 301 },
+      { "source": "/guides/language/common-prob", "destination": "/deprecated/sound-problems", "type": 301 },
       { "source": "/guides/language/concurrency", "destination": "/language/concurrency", "type": 301 },
       { "source": "/guides/language/effective-dart", "destination": "/effective-dart", "type": 301 },
       { "source": "/guides/language/effective-dart/:page*", "destination": "/effective-dart/:page*", "type": 301 },
@@ -226,10 +227,12 @@
       { "source": "/guides/language/numbers", "destination": "/resources/language/number-representation", "type": 301 },
       { "source": "/guides/language/sound-dart", "destination": "/language/type-system", "type": 301 },
       { "source": "/guides/language/sound-faq", "destination": "/language/type-system", "type": 301 },
+      { "source": "/guides/language/sound-problems", "destination": "/deprecated/sound-problems", "type": 301 },
       { "source": "/guides/language/spec", "destination": "/resources/language/spec", "type": 301 },
       { "source": "/guides/language/specifications", "destination": "/resources/language/spec", "type": 301 },
       { "source": "/guides/language/specifications/:page*", "destination": "/resources/language/spec/versions/:page*", "type": 301 },
       { "source": "/guides/language/type-system", "destination": "/language/type-system", "type": 301 },
+      { "source": "/guides/language/**", "destination": "/language", "type": 301 },
       { "source": "/guides/libraries", "destination": "/libraries", "type": 301 },
       { "source": "/guides/libraries/create-library-packages", "destination": "/tools/pub/create-packages", "type": 301 },
       { "source": "/guides/libraries/create-packages", "destination": "/tools/pub/create-packages", "type": 301 },
@@ -263,7 +266,7 @@
       { "source": "/keyword/class", "destination": "/language/classes#instance-variables", "type": 301 },
       { "source": "/keyword/const", "destination": "/language/variables#final-and-const", "type": 301 },
       { "source": "/keyword/continue", "destination": "/language/loops#break-and-continue", "type": 301 },
-      { "source": "/keyword/covariant", "destination": "/guides/language/sound-problems#the-covariant-keyword", "type": 301 },
+      { "source": "/keyword/covariant", "destination": "/deprecated/sound-problems#the-covariant-keyword", "type": 301 },
       { "source": "/keyword/default", "destination": "/language/branches#switch", "type": 301 },
       { "source": "/keyword/deferred", "destination": "/language/libraries#lazily-loading-a-library", "type": 301 },
       { "source": "/keyword/do", "destination": "/language/loops#while-and-do-while", "type": 301 },

src/_data/glossary.yml

@@ -614,7 +614,7 @@
     - text: "Covariance and contravariance"
       link: "https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)"
     - text: "The covariant keyword"
-      link: "/guides/language/sound-problems#the-covariant-keyword"
+      link: "/deprecated/sound-problems#the-covariant-keyword"
   labels:
     - "language"
     - "type system"

src/_data/keywords.yml

@@ -35,7 +35,7 @@
   link: /language/loops#break-and-continue
   type: reserved
 - term: 'covariant'
-  link: /guides/language/sound-problems#the-covariant-keyword
+  link: /deprecated/sound-problems#the-covariant-keyword
   type: bit
 - term: 'default'
   link: /language/branches#switch

src/_data/side-nav.yml

@@ -327,7 +327,7 @@
       - title: Customizing static analysis
         permalink: /tools/analysis
       - title: Fixing common type problems
-        permalink: /guides/language/sound-problems
+        permalink: /deprecated/sound-problems
       - title: Fixing type promotion failures
         permalink: /tools/non-promotion-reasons
       - title: Linter rules

src/_includes/navigation-main.html

@@ -15,7 +15,7 @@
     </li>
     <li class="mainnav__get-started">
       <a href="/guides" class="nav-link
-        {%- if page_url_path contains '/*/guides/' or page_url_path contains '/*/language/' or page_url_path contains '/*/resources/' or page_url_path contains '/*/codelabs/' or page_url_path contains '/*/samples/' or page_url_path contains '/*/tools/' or page_url_path contains '/*/tutorials/' or page_url_path contains '/*/server/' or page_url_path contains '/*/web/' or page_url_path contains '/*/effective-dart/' or page_url_path contains '/*/libraries/' %} active {%- endif -%}">
+        {%- if page_url_path contains '/*/guides/' or page_url_path contains '/*/language/' or page_url_path contains '/*/resources/' or page_url_path contains '/*/codelabs/' or page_url_path contains '/*/deprecated/' or page_url_path contains '/*/tools/' or page_url_path contains '/*/tutorials/' or page_url_path contains '/*/server/' or page_url_path contains '/*/web/' or page_url_path contains '/*/effective-dart/' or page_url_path contains '/*/libraries/' %} active {%- endif -%}">
         <span>Docs</span>
       </a>
     </li>

src/content/language/extend.md

@@ -78,7 +78,7 @@ it's similar to a downcast in that it can cause a type error at runtime.
 Still, narrowing the type is possible
 if the code can guarantee that a type error won't occur.
 In this case, you can use the
-[`covariant` keyword](/guides/language/sound-problems#the-covariant-keyword)
+[`covariant` keyword](/deprecated/sound-problems#the-covariant-keyword)
 in a parameter declaration.
 For details, see the
 [Dart language specification][].

src/content/language/type-system.md

@@ -179,7 +179,7 @@ subtype of the original parameter.
 
 :::note
 If you have a valid reason to use a subtype, you can use the
-[`covariant` keyword](/guides/language/sound-problems#the-covariant-keyword).
+[`covariant` keyword](/deprecated/sound-problems#the-covariant-keyword).
 :::
 
 Consider the `chase(Animal)` method for the `Animal` class:
@@ -488,7 +488,7 @@ and [Use sound parameter types when overriding methods](#use-proper-param-types)
 
 The following resources have further information on sound Dart:
 
-* [Fixing common type problems](/guides/language/sound-problems) - 
+* [Fixing common type problems](/deprecated/sound-problems) - 
   Errors you may encounter when writing sound Dart code, and how to fix them.
 * [Fixing type promotion failures](/tools/non-promotion-reasons) - 
   Understand and learn how to fix type promotion errors.

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

@@ -122,7 +122,7 @@ consider themselves as equal.
 :::
 
 [Built-in types]: /language/built-in-types
-[Dart Language Tour]: /guides/language
+[Dart Language Tour]: /language
 
 For example:
 The equals operator `==` and the `identical()` method return `true`

src/content/resources/whats-new.md

@@ -548,7 +548,7 @@ we made the following changes to this site:
 * Added a section on customizing [`dart fix`][].
 
 [Objective-C and Swift interop]: /interop/objective-c-interop
-[Fixing common type problems]: /guides/language/sound-problems
+[Fixing common type problems]: /deprecated/sound-problems
 [What not to commit]: /tools/pub/private-files
 [`dart pub get` Options]: /tools/pub/cmd/pub-get#options
 [`dart compile`]: /tools/dart-compile