Fix for IE9 html/htmlcollection test with Safe HTML

tools/dom/src/Validators.dart

Index: tools/dom/src/Validators.dart
diff --git a/tools/dom/src/Validators.dart b/tools/dom/src/Validators.dart
index 9572187666fa3aceefd10ec344f7151b16cf8e9b..430658edc35992da95c04503832b9ca5cf74266a 100644
--- a/tools/dom/src/Validators.dart
+++ b/tools/dom/src/Validators.dart
@@ -182,7 +182,8 @@ class _ValidatingTreeSanitizer implements NodeTreeSanitizer {
         var keys = attrs.keys.toList();
         for (var i = attrs.length - 1; i >= 0; --i) {
           var name = keys[i];
-          if (!validator.allowsAttribute(element, name, attrs[name])) {
+          if (!validator.allowsAttribute(element, name.toLowerCase(),
+              attrs[name])) {
             attrs.remove(name);
           }
         }