[flutter_markdown] Do not modify children of an Element after constru…

…ction (flutter#2758)

Do not modify children of a markdown Element after construction

The children list of an Element became unmodifiable in dart-lang/markdown#463, so this just changes flutter_markdown to not modify the children, in a test.

Fixes https://github.com/dart-lang/markdown/issues/480

packages/flutter_markdown/test/custom_syntax_test.dart

@@ -160,9 +160,10 @@ class WikilinkSyntax extends md.InlineSyntax {
 
   @override
   bool onMatch(md.InlineParser parser, Match match) {
-    final md.Element el = md.Element.withTag('wikilink');
-    el.attributes['href'] = match[1]!.replaceAll(' ', '_');
-    el.children!.add(md.Element.text('span', match[1]!));
+    final String link = match[1]!;
+    final md.Element el =
+        md.Element('wikilink', <md.Element>[md.Element.text('span', link)])
+          ..attributes['href'] = link.replaceAll(' ', '_');
 
     parser.addNode(el);
     return true;