Make sanitizer tests handle IE11, where tagName can't be clobbered

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index c272f55c8658ef392d1e094b9231b54caa8eb409..2e7a051bbdd51f21ce560c3c466b8d7062c3931a 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -40752,6 +40752,9 @@ class _ValidatingTreeSanitizer implements NodeTreeSanitizer {
     var elementTagName = 'element tag unavailable';
     try {
       elementTagName = element.tagName;
+      if (elementTagName != elementTagName.toString() {
+        corrupted = true; // The tag name is not a string
+      }
     } catch(e) {}
     _sanitizeElement(element, parent, corrupted, elementText, elementTagName,
         attrs, isAttr);
@@ -40807,7 +40810,7 @@ class _ValidatingTreeSanitizer implements NodeTreeSanitizer {
   void sanitizeNode(Node node, Node parent) {
     switch (node.nodeType) {
       case Node.ELEMENT_NODE:
-        _sanitizeUntrustedElement(node, parent); 
+        _sanitizeUntrustedElement(node, parent);
         break;
       case Node.COMMENT_NODE:
       case Node.DOCUMENT_FRAGMENT_NODE: