Remove dart backend

test/dart_codegen/expect/core/set.dart

-part of dart.core;
- abstract class Set<E> extends IterableBase<E> implements EfficientLength {factory Set() = LinkedHashSet<E>;
- factory Set.identity() = LinkedHashSet<E>.identity;
- factory Set.from(Iterable elements) = LinkedHashSet<E>.from;
- Iterator<E> get iterator;
- bool contains(Object value);
- bool add(E value);
- void addAll(Iterable<E> elements);
- bool remove(Object value);
- E lookup(Object object);
- void removeAll(Iterable<Object> elements);
- void retainAll(Iterable<Object> elements);
- void removeWhere(bool test(E element));
- void retainWhere(bool test(E element));
- bool containsAll(Iterable<Object> other);
- Set<E> intersection(Set<Object> other);
- Set<E> union(Set<E> other);
- Set<E> difference(Set<Object> other);
- void clear();
- Set<E> toSet();
-}