message: Return DefaultCustomSectionsIterator for custom_sections
diff: @@ -560,7 +560,16 @@ impl Module {
  /// is returned.
  pub fn custom_sections<'a>(&'a self, name: &'a str) -> impl Iterator<Item = Box<[u8]>> + 'a {
  // TODO: implement on JavaScript
  - unimplemented!()
  + DefaultCustomSectionsIterator { }
  + }
  +}
  +
  +pub struct DefaultCustomSectionsIterator { }
  +
  +impl Iterator for DefaultCustomSectionsIterator {
  + type Item = Box<[u8]>;
  + fn next(&mut self) -> Option<Self::Item> {
  + None
  }
  }
  