Reapply "Remove Object.dynamic."

utils/template/htmltree.dart

Index: utils/template/htmltree.dart
diff --git a/utils/template/htmltree.dart b/utils/template/htmltree.dart
index ad941f7f8b38cc70fbe3255fb274604cb39372de..e243d1d0e50f0fae6308cdd7db8a9ed267ef2f01 100644
--- a/utils/template/htmltree.dart
+++ b/utils/template/htmltree.dart
@@ -361,9 +361,10 @@ class TreePrinter implements TreeVisitor {
   void visitTemplateDocument(TemplateDocument node) {
     output.heading('Content', node.span);
     output.depth++;
-    // TODO(terry): Ugly use dynamic[0] instead children[0] to surpress warning.
+    // TODO(terry): Ugly use of 'as Dynamic' instead of children[0] to
+    //              surpress warning.
     assert(node.children.length == 1 &&
-        node.children.dynamic[0].tagTokenId == -1);
+        (node.children as Dynamic)[0].tagTokenId == -1);
     output.writeNodeList("document", node.children);
     output.depth--;
   }