[messages] Change check_all_yaml.dart to match lints ignoring case.

Now that lint name matching is case insensitive
(https://dart-review.googlesource.com/c/sdk/+/465964), it makes sense
for `check_all_yaml.dart` to ignore case when checking the contents of
the file `pkg/linter/example/all.yaml`.

This paves the way for a follow-up CL that will change the names
declared in the linter to all lower case.

Change-Id: I6a6a6964b0b6863fbbf9998f0333863e2c15f60d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/465990
Reviewed-by: Samuel Rawlins <[email protected]>
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: stereotype441

pkg/linter/tool/checks/check_all_yaml.dart

@@ -34,7 +34,10 @@ String? checkAllYaml() {
     return "Error: '$allYamlPath' does not have a 'linter' section.";
   }
 
-  var configuredRules = (linterSection['rules'] as YamlList?)?.cast<String>();
+  var configuredRules = (linterSection['rules'] as YamlList?)
+      ?.cast<String>()
+      .map((s) => s.toLowerCase())
+      .toList();
   if (configuredRules == null) {
     return "Error: '$allYamlPath' does not have a 'rules' section.";
   }
@@ -55,7 +58,7 @@ String? checkAllYaml() {
         (r) =>
             !r.state.isDeprecated && !r.state.isInternal && !r.state.isRemoved,
       )
-      .map((r) => r.name);
+      .map((r) => r.name.toLowerCase());
 
   var extraRules = <String>[];
   var missingRules = <String>[];