Fix HtmlBlockSyntax (dart-lang/markdown#548)

diff --git a/pkgs/markdown/lib/src/block_syntaxes/html_block_syntax.dart b/pkgs/markdown/lib/src/block_syntaxes/html_block_syntax.dart
index ff00850..c1aa85c 100644
--- a/pkgs/markdown/lib/src/block_syntaxes/html_block_syntax.dart
+++ b/pkgs/markdown/lib/src/block_syntaxes/html_block_syntax.dart

@@ -73,12 +73,9 @@
       parser.advance();
     }
 
-    // If the following lines start an HTML block again, put them together with
-    // current HTML block.
-    if (!parser.isDone &&
-        parser.next != null &&
-        pattern.hasMatch(parser.next!.content)) {
-      parser.advance();
+    // If the current line start an HTML block again, put them together with
+    // the previous HTML block.
+    if (!parser.isDone && pattern.hasMatch(parser.current.content)) {
       lines.addAll(parseChildLines(parser));
     }
 

diff --git a/pkgs/markdown/test/original/html_block.unit b/pkgs/markdown/test/original/html_block.unit
new file mode 100644
index 0000000..f6b9216
--- /dev/null
+++ b/pkgs/markdown/test/original/html_block.unit

@@ -0,0 +1,11 @@
+>>> issue https://github.com/dart-lang/markdown/issues/547
+<?code-excerpt ?>
+```xml
+<q>
+</q>
+```
+<<<
+<?code-excerpt ?>
+<pre><code class="language-xml">&lt;q&gt;
+&lt;/q&gt;
+</code></pre>
\ No newline at end of file