comment out excerpt, add version note

Author: MaryaBelanger

examples/misc/lib/language_tour/built_in_types.dart

@@ -1,6 +1,5 @@
 // ignore_for_file: dead_code, unused_local_variable, type_annotate_public_apis
 // ignore_for_file: prefer_single_quotes, prefer_collection_literals
-// ignore_for_file: experiment_not_enabled
 
 void miscDeclAnalyzedButNotTested() {
   {
@@ -38,15 +37,16 @@ void miscDeclAnalyzedButNotTested() {
     // #enddocregion const-num
   }
 
-  {
-    // #docregion digit-separators
-    var n1 = 1_000_000;
-    var n2 = 0.000_000_000_01;
-    var n3 = 0x00_14_22_01_23_45;  // MAC address
-    var n4 = 555_123_4567;  // US Phone number
-    var n5 = 100__000_000__000_000;  // one hundred million million!
-    // #enddocregion digit-separators
-  }
+  // Uncomment when feature is stable:
+  // {
+  //   // #docregion digit-separators
+  //   var n1 = 1_000_000;
+  //   var n2 = 0.000_000_000_01;
+  //   var n3 = 0x00_14_22_01_23_45;  // MAC address
+  //   var n4 = 555_123_4567;  // US Phone number
+  //   var n5 = 100__000_000__000_000;  // one hundred million million!
+  //   // #enddocregion digit-separators
+  // }
 
   {
     // #docregion quoting

src/content/language/built-in-types.md

@@ -177,7 +177,11 @@ You can use one or more underscores (`_`) as digit separators
 to make long number literals more readable.
 Multiple digit separators allow for higher level grouping.
 
+{% comment %}
+Uncomment when feature is stable:
+
 <?code-excerpt "misc/lib/language_tour/built_in_types.dart (digit-separators)"?>
+{% endcomment %}
 ```dart
 var n1 = 1_000_000;
 var n2 = 0.000_000_000_01;
@@ -186,6 +190,10 @@ var n4 = 555_123_4567;  // US Phone number
 var n5 = 100__000_000__000_000;  // one hundred million million!
 ```
 
+:::version-note
+Using digit separators requires a [language version][] of at least 3.6.0.
+:::
+
 ## Strings
 
 A Dart string (`String` object) holds a sequence of UTF-16 code units.
@@ -427,3 +435,4 @@ Symbol literals are compile-time constants.
 [characters API]: {{site.pub-api}}/characters
 [characters example]: {{site.pub-pkg}}/characters/example
 [`Symbol`]: {{site.dart-api}}/{{site.sdkInfo.channel}}/dart-core/Symbol-class.html
+[language version]: /guides/language/evolution#language-versioning