Adding warning message when HTML contents get sanitized away.

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 3d25f4253fe5f1afe65030b0ba3f6e348664ecb7..dc3fb99842a2f43ac1a48ffbec7cee523bfccb35 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -26098,13 +26098,13 @@ class Url extends NativeFieldWrapperClass1 {
     if ((blob_OR_source_OR_stream is Blob || blob_OR_source_OR_stream == null)) {
       return _createObjectURL_1(blob_OR_source_OR_stream);
     }
-    if ((blob_OR_source_OR_stream is MediaSource || blob_OR_source_OR_stream == null)) {
+    if ((blob_OR_source_OR_stream is MediaStream || blob_OR_source_OR_stream == null)) {
       return _createObjectURL_2(blob_OR_source_OR_stream);
     }
-    if ((blob_OR_source_OR_stream is _WebKitMediaSource || blob_OR_source_OR_stream == null)) {
+    if ((blob_OR_source_OR_stream is MediaSource || blob_OR_source_OR_stream == null)) {
       return _createObjectURL_3(blob_OR_source_OR_stream);
     }
-    if ((blob_OR_source_OR_stream is MediaStream || blob_OR_source_OR_stream == null)) {
+    if ((blob_OR_source_OR_stream is _WebKitMediaSource || blob_OR_source_OR_stream == null)) {
       return _createObjectURL_4(blob_OR_source_OR_stream);
     }
     throw new ArgumentError("Incorrect number or type of arguments");
@@ -33792,6 +33792,8 @@ class _ValidatingTreeSanitizer implements NodeTreeSanitizer {
         Element element = node;
         var attrs = element.attributes;
         if (!validator.allowsElement(element)) {
+          window.console.warn(
+              'Removing disallowed element <${element.tagName}>');
           element.remove();
           break;
         }
@@ -33799,6 +33801,8 @@ class _ValidatingTreeSanitizer implements NodeTreeSanitizer {
         var isAttr = attrs['is'];
         if (isAttr != null) {
           if (!validator.allowsAttribute(element, 'is', isAttr)) {
+            window.console.warn('Removing disallowed type extension '
+                '<${element.tagName} is="$isAttr">');
             element.remove();
             break;
           }
@@ -33811,6 +33815,8 @@ class _ValidatingTreeSanitizer implements NodeTreeSanitizer {
           var name = keys[i];
           if (!validator.allowsAttribute(element, name.toLowerCase(),
               attrs[name])) {
+            window.console.warn('Removing disallowed attribute '
+                '<${element.tagName} $name="${attrs[name]}">');
             attrs.remove(name);
           }
         }