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

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 2063e0bcdf505a650d62affe79caffc733c7d090..e1ce5b66792c6be8779b37b4982c959607f863ca 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -40822,6 +40822,9 @@ 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);
@@ -40877,7 +40880,7 @@ 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: