Fix assert in dartdoc_checker and enable asserts (#64111)

diff --git a/dev/tools/dartdoc_checker.dart b/dev/tools/dartdoc_checker.dart
index 686e918..e91ad1b 100644
--- a/dev/tools/dartdoc_checker.dart
+++ b/dev/tools/dartdoc_checker.dart
@@ -84,7 +84,7 @@
 }
 
 int _scanFile(File file) {
-  assert(path.extension(file.path) == 'html');
+  assert(path.extension(file.path) == '.html');
   final Iterable<String> matches = _pattern.allMatches(file.readAsStringSync())
       .map((RegExpMatch m ) => m.group(0));