Fix dart-lang/markdown#586: encode image tag's src attribute (dart-archive/markdown#589)

Author: tomyeh

pkgs/markdown/lib/src/inline_syntaxes/image_syntax.dart

@@ -24,7 +24,9 @@ class ImageSyntax extends LinkSyntax {
   }) {
     final element = Element.empty('img');
     final children = getChildren();
-    element.attributes['src'] = destination;
+    element.attributes['src'] = normalizeLinkDestination(
+      escapePunctuation(destination),
+    );
     element.attributes['alt'] = children.map((node) {
       // See https://spec.commonmark.org/0.30/#image-description.
       // An image description may contain links. Fetch text from the alt

pkgs/markdown/test/original/inline_images.unit

@@ -18,3 +18,8 @@
 
 <<<
 <p><img src="http://foo.com/foo.png" alt="alt" /></p>
+>>> XSS
+![Uh oh...]("onerror="alert('XSS'))
+
+<<<
+<p><img src="%22onerror=%22alert('XSS')" alt="Uh oh..." /></p>