Big merge from experimental to bleeding edge.

sdk/lib/_internal/dartdoc/lib/src/markdown/html_renderer.dart

Index: sdk/lib/_internal/dartdoc/lib/src/markdown/html_renderer.dart
diff --git a/sdk/lib/_internal/dartdoc/lib/src/markdown/html_renderer.dart b/sdk/lib/_internal/dartdoc/lib/src/markdown/html_renderer.dart
index d44c5f51e4729bfd6ade794f5f9a0cd746207f81..1695e96b7d3e99fb09273a335b1e019afa148a92 100644
--- a/sdk/lib/_internal/dartdoc/lib/src/markdown/html_renderer.dart
+++ b/sdk/lib/_internal/dartdoc/lib/src/markdown/html_renderer.dart
@@ -39,7 +39,7 @@ class HtmlRenderer implements NodeVisitor {
     // Sort the keys so that we generate stable output.
     // TODO(rnystrom): This assumes keys returns a fresh mutable
     // collection.
-    final attributeNames = element.attributes.keys;
+    final attributeNames = element.attributes.keys.toList();
     attributeNames.sort((a, b) => a.compareTo(b));
     for (final name in attributeNames) {
       buffer.add(' $name="${element.attributes[name]}"');