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..14b7bb03fd0b72a779dab12c1746c635f89ea5f7 100644
--- a/utils/dartdoc/html_renderer.dart
+++ b/utils/dartdoc/html_renderer.dart
@@ -34,7 +34,12 @@ class HtmlRenderer implements NodeVisitor {
 
     buffer.add('<${element.tag}');
 
-    for (final name in element.attributes.getKeys()) {
+    // Sort the keys so that we generate stable output.
+    // TODO(rnystrom): This assumes getKeys() returns a fresh mutable
+    // collection.
+    final attributeNames = element.attributes.getKeys();
+    attributeNames.sort((a, b) => a.compareTo(b));
+    for (final name in attributeNames) {
       buffer.add(' $name="${element.attributes[name]}"');
     }