Remove dart backend

test/dart_codegen/expect/collection/linked_hash_set.dart

-part of dart.collection;
- abstract class LinkedHashSet<E> implements HashSet<E> {external factory LinkedHashSet({
-  bool equals(E e1, E e2), int hashCode(E e), bool isValidKey(potentialKey)}
-);
- external factory LinkedHashSet.identity();
- factory LinkedHashSet.from(Iterable<E> elements) {
-  LinkedHashSet<E> result = new LinkedHashSet<E>();
-   for (final E element in elements) {
-    result.add(element);
-    }
-   return result;
-  }
- void forEach(void action(E element));
- Iterator<E> get iterator;
-}