Revert "Move dart2js from sdk/lib/_internal/compiler to pkg/compiler"

pkg/compiler/lib/src/util/emptyset.dart

-// Copyright (c) 2014, 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.
-
-library dart2js.util.emptyset;
-
-import 'dart:collection' show IterableBase;
-
-class ImmutableEmptySet<E> extends IterableBase<E> implements Set<E> {
-  const ImmutableEmptySet();
-
-  get iterator => const _EmptySetIterator();
-  int get length => 0;
-  bool get isEmpty => true;
-
-  get _immutableError => throw new UnsupportedError("EmptySet is immutable");
-
-  bool add (E element) => _immutableError;
-  void addAll(Iterable<E> elements) => _immutableError;
-
-  E lookup(E element) => null;
-  bool remove(E element) => false;
-  void removeAll(Iterable<E> elements) {}
-  void removeWhere(bool test(E element)) {}
-  void retainAll(Iterable<E> elements) {}
-  void retainWhere(bool test(E element)) {}
-  void forEach(void action(E element)) {}
-  void clear() {}
-
-  bool contains(E element) => false;
-  bool containsAll(Iterable<E> other) => other.isEmpty;
-
-  Set<E> union(Set<E> other) => new Set.from(other);
-  Set<E> intersection(Set<E> other) => this;
-  Set<E> difference(Set<E> other) => this;
-  Set<E> toSet() => new Set();
-}
-
-class _EmptySetIterator<E> implements Iterator<E> {
-  const _EmptySetIterator();
-
-  E get current => null;
-  bool moveNext() => false;
-}