Add markdown to third_party/pkg

third_party/pkg/markdown/test/markdown_test.dart

Index: third_party/pkg/markdown/test/markdown_test.dart
diff --git a/sdk/lib/_internal/dartdoc/test/markdown_test.dart b/third_party/pkg/markdown/test/markdown_test.dart
similarity index 97%
copy from sdk/lib/_internal/dartdoc/test/markdown_test.dart
copy to third_party/pkg/markdown/test/markdown_test.dart
index aff43da3dfdeeee4cbd280d64cbf8749edd8c657..fcf76d13165d9bc93fd08d2dfcb3726ee024626e 100644
--- a/sdk/lib/_internal/dartdoc/test/markdown_test.dart
+++ b/third_party/pkg/markdown/test/markdown_test.dart
@@ -424,6 +424,35 @@ void main() {
         ''');
   });
 
+  group('Fenced code blocks', () {
+    validate('without an optional language identifier', '''
+        ```
+        code
+        ```
+        ''', '''
+        <pre><code>code
+        </code></pre>
+        ''');
+
+    validate('with an optional language identifier', '''
+        ```dart
+        code
+        ```
+        ''', '''
+        <pre><code>code
+        </code></pre>
+        ''');
+
+    validate('escape HTML characters', '''
+        ```
+        <&>
+        ```
+        ''', '''
+        <pre><code>&lt;&amp;&gt;
+        </code></pre>
+        ''');
+  });
+
   group('Horizontal rules', () {
     validate('from dashes', '''
         ---