Reapply 12799.

lib/html/src/dart2js_Conversions.dart

Index: lib/html/src/dart2js_Conversions.dart
===================================================================
--- lib/html/src/dart2js_Conversions.dart	(revision 12802)
+++ lib/html/src/dart2js_Conversions.dart	(working copy)
@@ -205,14 +205,11 @@
     }
 
     if (e is _FileListImpl) return e;
-    if (e is FileList) {
-      throw const NotImplementedException('structured clone of FileList');
-    }
 
     // TODO(sra): Firefox: How to convert _TypedImageData on the other end?
     if (e is _ImageDataImpl) return e;
     if (e is ImageData) {
-      throw const NotImplementedException('structured clone of FileList');
+      throw const NotImplementedException('structured clone of ImageData');
     }
 
     if (e is _ArrayBufferImpl) return e;