visitText to use textContent getter so subclassing Text class works correctly. (dart-lang/markdown#431)

pkgs/markdown/lib/src/html_renderer.dart

diff --git a/pkgs/markdown/lib/src/html_renderer.dart b/pkgs/markdown/lib/src/html_renderer.dart
index f317707..a6abfcd 100644
--- a/pkgs/markdown/lib/src/html_renderer.dart
+++ b/pkgs/markdown/lib/src/html_renderer.dart
@@ -103,13 +103,13 @@
 
   @override
   void visitText(Text text) {
-    var content = text.text;
+    var content = text.textContent;
     if (const ['br', 'p', 'li'].contains(_lastVisitedTag)) {
       final lines = LineSplitter.split(content);
       content = content.contains('<pre>')
           ? lines.join('\n')
           : lines.map((line) => line.trimLeft()).join('\n');
-      if (text.text.endsWith('\n')) {
+      if (text.textContent.endsWith('\n')) {
         content = '$content\n';
       }
     }