Change getChildren parameter of _resolveReferenceLink to required and remove unnecessary non null assertion (dart-lang/markdown#399)

pkgs/markdown/lib/src/inline_parser.dart

diff --git a/pkgs/markdown/lib/src/inline_parser.dart b/pkgs/markdown/lib/src/inline_parser.dart
index a0ea9d5..d14e8fe 100644
--- a/pkgs/markdown/lib/src/inline_parser.dart
+++ b/pkgs/markdown/lib/src/inline_parser.dart
@@ -1043,11 +1043,11 @@
   /// [label] does not need to be normalized.
   Node? _resolveReferenceLink(
       String label, Map<String, LinkReference> linkReferences,
-      {List<Node> Function()? getChildren}) {
+      {required List<Node> Function() getChildren}) {
     var linkReference = linkReferences[normalizeLinkLabel(label)];
     if (linkReference != null) {
       return _createNode(linkReference.destination, linkReference.title,
-          getChildren: getChildren!);
+          getChildren: getChildren);
     } else {
       // This link has no reference definition. But we allow users of the
       // library to specify a custom resolver function ([linkResolver]) that
@@ -1062,7 +1062,7 @@
           .replaceAll(r'\[', '[')
           .replaceAll(r'\]', ']'));
       if (resolved != null) {
-        getChildren!();
+        getChildren();
       }
       return resolved;
     }