Hook up the dartdoc tests to test.py, test.dart, and frog/presubmit.py.

utils/dartdoc/html_renderer.dart

Index: utils/dartdoc/html_renderer.dart
diff --git a/utils/dartdoc/html_renderer.dart b/utils/dartdoc/html_renderer.dart
index ec4bebb1d00316bb270b484f635f59d5fdfe4490..1665d86aebe7a85cc3e8c0a78e52ce0b8f80fc83 100644
--- a/utils/dartdoc/html_renderer.dart
+++ b/utils/dartdoc/html_renderer.dart
@@ -34,7 +34,10 @@ class HtmlRenderer implements NodeVisitor {
 
     buffer.add('<${element.tag}');
 
-    for (final name in element.attributes.getKeys()) {
+    // Sort the keys so that we generate stable output.
+    final attributeNames = new List.from(element.attributes.getKeys());
+    attributeNames.sort((a, b) => a.compareTo(b));
+    for (final name in attributeNames) {
       buffer.add(' $name="${element.attributes[name]}"');
     }