Remove dart backend

test/dart_codegen/expect/collection/hash_set.dart

-part of dart.collection;
- abstract class _HashSetBase<E> extends SetBase<E> {Set<E> difference(Set<Object> other) {
-  Set<E> result = _newSet();
-   for (var element in this) {
-    if (!other.contains(element)) result.add(element);
-    }
-   return result;
-  }
- Set<E> intersection(Set<Object> other) {
-  Set<E> result = _newSet();
-   for (var element in this) {
-    if (other.contains(element)) result.add(element);
-    }
-   return result;
-  }
- Set<E> _newSet();
- Set<E> toSet() => _newSet()..addAll(this);
-}
- abstract class HashSet<E> implements Set<E> {external factory HashSet({
-bool equals(E e1, E e2), int hashCode(E e), bool isValidKey(potentialKey)}
-);
- external factory HashSet.identity();
- factory HashSet.from(Iterable elements) {
-HashSet<E> result = new HashSet<E>();
- for (E e in DEVC$RT.cast(elements, DEVC$RT.type((Iterable<dynamic> _) {
-  }
-), DEVC$RT.type((Iterable<E> _) {
-  }
-), "CompositeCast", """line 103, column 17 of dart:collection/hash_set.dart: """, elements is Iterable<E>, false)) result.add(e);
- return result;
-}
- Iterator<E> get iterator;
-}