FileList is a sequence - make it have indexer.

lib/dom/idl/dart/dart.idl

 
 // This file introduces / supplements and forces Dart declarations.
 
 module default {
+  FileList implements sequence<File>;
+  HTMLCollection implements sequence<Node>;
+  MediaList implements sequence<DOMString>;
   NamedNodeMap implements sequence<Node>;
   NodeList implements sequence<Node>;
-  HTMLCollection implements sequence<Node>;
-  MediaList implements sequence<DOMString>;
   StyleSheetList implements sequence<StyleSheet>;
   TouchList implements sequence<Touch>;
 
   Float32Array implements sequence<double>;
   Float64Array implements sequence<double>;
   Int8Array implements sequence<int>;
   Int16Array implements sequence<int>;
   Int32Array implements sequence<int>;
   Uint8Array implements sequence<int>;
   // Ugliness ahead: we cannot analyse inheritance right now when deducing if
@@ skipping 218 matching lines @@
   [Supplemental, Callback] // Add missing Callback attribute.
   interface VoidCallback {
   };
 };
 
 module svg {
     interface SVGNumber {
       [StrictTypeChecking, Custom] attribute double value;
     };
 }