Update the ones related to Element.children (dart-lang/markdown#405)

pkgs/markdown/lib/src/ast.dart

diff --git a/pkgs/markdown/lib/src/ast.dart b/pkgs/markdown/lib/src/ast.dart
index 25b675b..182fc4c 100644
--- a/pkgs/markdown/lib/src/ast.dart
+++ b/pkgs/markdown/lib/src/ast.dart
@@ -55,7 +55,7 @@
 
   @override
   String get textContent {
-    return (children ?? []).map((Node? child) => child!.textContent).join('');
+    return (children ?? []).map((child) => child.textContent).join('');
   }
 }
 

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 bdf6612..88896e0 100644
--- a/pkgs/markdown/lib/src/inline_parser.dart
+++ b/pkgs/markdown/lib/src/inline_parser.dart
@@ -285,7 +285,7 @@
 
   // Combine any remaining adjacent Text nodes. This is important to produce
   // correct output across newlines, where whitespace is sometimes compressed.
-  void _combineAdjacentText(List<Node?> nodes) {
+  void _combineAdjacentText(List<Node> nodes) {
     for (var i = 0; i < nodes.length - 1; i++) {
       var node = nodes[i];
       if (node is Element && node.children != null) {
@@ -294,10 +294,10 @@
       }
       if (node is Text && nodes[i + 1] is Text) {
         var buffer =
-            StringBuffer('${node.textContent}${nodes[i + 1]!.textContent}');
+            StringBuffer('${node.textContent}${nodes[i + 1].textContent}');
         var j = i + 2;
         while (j < nodes.length && nodes[j] is Text) {
-          buffer.write(nodes[j]!.textContent);
+          buffer.write(nodes[j].textContent);
           j++;
         }
         nodes[i] = Text(buffer.toString());