Make ExtensionSet static fields final

pkgs/markdown/lib/src/extension_set.dart

diff --git a/pkgs/markdown/lib/src/extension_set.dart b/pkgs/markdown/lib/src/extension_set.dart
index 6c32627..cb381de 100644
--- a/pkgs/markdown/lib/src/extension_set.dart
+++ b/pkgs/markdown/lib/src/extension_set.dart
@@ -2,12 +2,12 @@
 import 'inline_parser.dart';
 
 class ExtensionSet {
-  static ExtensionSet none = new ExtensionSet._([], []);
+  static final ExtensionSet none = new ExtensionSet._([], []);
 
-  static ExtensionSet commonMark = new ExtensionSet._(
+  static final ExtensionSet commonMark = new ExtensionSet._(
       [const FencedCodeBlockSyntax()], [new InlineHtmlSyntax()]);
 
-  static ExtensionSet gitHub = new ExtensionSet._(
+  static final ExtensionSet gitHub = new ExtensionSet._(
       [const FencedCodeBlockSyntax(), const TableSyntax()],
       [new InlineHtmlSyntax()]);