Made index_db Cursor.next behavior consistent between JS and vm.

sdk/lib/indexed_db/dart2js/indexed_db_dart2js.dart

 /**
  * A client-side key-value store with support for indexes.
  *
  * Many browsers support IndexedDB—a web standard for
  * an indexed database.
  * By storing data on the client in an IndexedDB,
  * a web app gets some advantages, such as faster performance and persistence.
  * To find out which browsers support IndexedDB,
  * refer to [Can I Use?](http://caniuse.com/#feat=indexeddb)
  *
@@ skipping 213 matching lines @@
 
   @DomName('IDBCursor.value')
   Future update(value) {
    try {
       return _completeRequest(_update(value));
     } catch (e, stacktrace) {
       return new Future.error(e, stacktrace);
     }
   }
 
+  @JSName('continue')
+  @DomName('IDBCursor.continue')
+  void next([Object key]) {
+    if (key == null) {
+      JS('void', '#.continue()', this);
+    } else {
+      JS('void', '#.continue(#)', this, key);
+    }
+  }
   
   @DomName('IDBCursor.direction')
   @DocsEditable()
   final String direction;
 
   @DomName('IDBCursor.key')
   @DocsEditable()
   @_annotation_Creates_IDBKey
   @_annotation_Returns_IDBKey
   final Object key;
@@ skipping 12 matching lines @@
 
   @DomName('IDBCursor.advance')
   @DocsEditable()
   void advance(int count) native;
 
   @JSName('delete')
   @DomName('IDBCursor.delete')
   @DocsEditable()
   Request _delete() native;
 
-  @JSName('continue')
-  @DomName('IDBCursor.continue')
-  @DocsEditable()
-  void next([Object key]) native;
-
   @DomName('IDBCursor.update')
   @DocsEditable()
   Request _update(/*any*/ value) {
     var value_1 = convertDartToNative_SerializedScriptValue(value);
     return _update_1(value_1);
   }
   @JSName('update')
   @DomName('IDBCursor.update')
   @DocsEditable()
   Request _update_1(value) native;
@@ skipping 1058 matching lines @@
 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 
 @DocsEditable()
 @DomName('IDBAny')
 @deprecated // nonstandard
 abstract class _IDBAny extends Interceptor native "IDBAny" {
 }