[messages] Use lowerCaseName in verify_docs.

Change the `verify_docs` package's use of `DiagnosticCode.name` to
`DiagnosticCode.lowerCaseName`. There is no functional change since
all diagnostic codes generated by the analyzer are already in lower
case.

This helps pave the way for eventually deprecating and removing
`DiagnosticCode.name`, which will help ensure that analyzer clients
treat diagnostic codes in a case-insensitive fashion.

Change-Id: I6a6a69645300e89b1bed342804d5e89ae9cf556b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/468141
Reviewed-by: Nate Bosch <nbosch@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

tools/verify_docs/bin/verify_docs.dart

@@ -372,7 +372,7 @@ class ValidateCommentCodeSamplesVisitor extends GeneralizingAstVisitor {
           final location = result.lineInfo.getLocation(diagnostic.offset);
           print(
             '  ${_severity(diagnostic.severity)}: ${diagnostic.message} '
-            '[$location] (${diagnostic.diagnosticCode.name.toLowerCase()})',
+            '[$location] (${diagnostic.diagnosticCode.lowerCaseName})',
           );
         }
         print('');