Fix release notes generator (#9671)

tool/lib/commands/release_notes_helper.dart

diff --git a/tool/lib/commands/release_notes_helper.dart b/tool/lib/commands/release_notes_helper.dart
index f70abc0..f98b984 100644
--- a/tool/lib/commands/release_notes_helper.dart
+++ b/tool/lib/commands/release_notes_helper.dart
@@ -141,19 +141,19 @@
       }
     }
 
-    releaseNotesMd.writeAsStringSync('''---
+    final metadataHeader = '''---
 title: DevTools $releaseNotesVersion release notes
 shortTitle: $releaseNotesVersion release notes
 breadcrumb: $releaseNotesVersion
 showToc: false
 ---
 
-''');
+''';
 
     // Write the contents of the 'release-notes-<x.y.z>.md' file,
     // including any updates for image paths.
     releaseNotesMd.writeAsStringSync(
-      srcLines.joinWithNewLine().trimLeft(),
+      metadataHeader + srcLines.joinWithNewLine().trimLeft(),
       flush: true,
     );