Adding warning message when HTML contents get sanitized away.

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 eb831c073f31928cb771e64492cb23d547e5829e..167809c0843f55c3cbae7c77fd69419e78ca656a 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -32333,6 +32333,8 @@ class _ValidatingTreeSanitizer implements NodeTreeSanitizer {
         Element element = node;
         var attrs = element.attributes;
         if (!validator.allowsElement(element)) {
+          window.console.warn(
+              'Removing disallowed element <${element.tagName}>');
           element.remove();
           break;
         }
@@ -32340,6 +32342,8 @@ class _ValidatingTreeSanitizer implements NodeTreeSanitizer {
         var isAttr = attrs['is'];
         if (isAttr != null) {
           if (!validator.allowsAttribute(element, 'is', isAttr)) {
+            window.console.warn('Removing disallowed type extension '
+                '<${element.tagName} is="$isAttr">');
             element.remove();
             break;
           }
@@ -32352,6 +32356,8 @@ class _ValidatingTreeSanitizer implements NodeTreeSanitizer {
           var name = keys[i];
           if (!validator.allowsAttribute(element, name.toLowerCase(),
               attrs[name])) {
+            window.console.warn('Removing disallowed attribute '
+                '<${element.tagName} $name="${attrs[name]}">');
             attrs.remove(name);
           }
         }