Add const UnmodifiableSetView.empty().

pkgs/collection/lib/src/unmodifiable_wrappers.dart

@@ -2,10 +2,12 @@
 // 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.
 
-export "dart:collection" show UnmodifiableListView, UnmodifiableMapView;
+import 'dart:collection' show IterableBase;
 
 import 'wrappers.dart';
 
+export "dart:collection" show UnmodifiableListView, UnmodifiableMapView;
+
 /// A fixed-length list.
 ///
 /// A `NonGrowableListView` contains a [List] object and ensures that
@@ -91,10 +93,116 @@ abstract class NonGrowableListMixin<E> implements List<E> {
 /// such as [add] and [remove], throw an [UnsupportedError].
 /// Permitted operations defer to the wrapped set.
 class UnmodifiableSetView<E> extends DelegatingSet<E>
-                             with UnmodifiableSetMixin<E> {
+    with UnmodifiableSetMixin<E> {
   UnmodifiableSetView(Set<E> setBase) : super(setBase);
+
+  /// Returns an unmodifiable empty set.
+  const factory UnmodifiableSetView.empty() = _EmptyUnmodifiableSet<E>;
+}
+
+/// An unmodifiable empty set that doesn't allow adding-operations.
+/// Removing-operations are ok, because the don't modify the contents of an
+/// empty set.
+// A super class with const constructor is needed, so IterableBase<E> fits.
+class _EmptyUnmodifiableSet<E> extends IterableBase<E>
+    implements UnmodifiableSetView<E> {
+  static /*=T*/ _throw/*<T>*/() {
+    throw new UnsupportedError(
+        "Cannot call modifying operations on an unmodifiable Set");
+  }
+
+  /// Returns an empty Iterator.
+  // new Iterabel.generate(0).iterator reduces to
+  // new EmptyIterable().iterator so this should be efficient.
+  Iterator<E> get _emptyIterator => new Iterable<E>.generate(0).iterator;
+
+  /// Returns always an empty Iterator;
+  @override
+  Iterator<E> get iterator => _emptyIterator;
+
+  /// Returns always 0;
+  /// The length of en empty Set fixed to 0.
+  @override
+  int get length => 0;
+
+  const _EmptyUnmodifiableSet();
+
+  /// Throws an [UnsupportedError];
+  /// modifying operations are disallowed on unmodifiable sets.
+  @override
+  bool add(E value) => _throw();
+
+  /// Throws an [UnsupportedError];
+  /// operations that add to the set are disallowed.
+  @override
+  void addAll(Iterable<E> elements) => _throw();
+
+  /// Throws an [UnsupportedError];
+  /// modifying operations are disallowed on unmodifiable sets.
+  @override
+  void clear() => _throw();
+
+  /// Returns always false;
+  /// The empty set doesn't contain any elements.
+  @override
+  bool contains(Object element) => false;
+
+  /// Returns true if other is empty, else false:
+  @override
+  bool containsAll(Iterable<Object> other) => other.isEmpty;
+
+  /// Returns always null;
+  /// The empty set doesn't contain any elements and therefore returns null.
+  @override
+  E lookup(Object element) => null;
+
+  /// Throws an [UnsupportedError];
+  /// modifying operations are disallowed on unmodifiable sets.
+  @override
+  bool remove(Object element) => _throw();
+
+  /// Throws an [UnsupportedError];
+  /// modifying operations are disallowed on unmodifiable sets.
+  @override
+  void removeAll(Iterable<Object> elements) => _throw();
+
+  /// Throws an [UnsupportedError];
+  /// modifying operations are disallowed on unmodifiable sets.
+  @override
+  void removeWhere(bool test(E element)) => _throw();
+
+  /// Throws an [UnsupportedError];
+  /// modifying operations are disallowed on unmodifiable sets.
+  @override
+  void retainWhere(bool test(E element)) => _throw();
+
+  /// Throws an [UnsupportedError];
+  /// modifying operations are disallowed on unmodifiable sets.
+  @override
+  void retainAll(Iterable<Object> elements) => _throw();
+
+  /// Returns itself because the behaviour (in regards to add(), remove())
+  /// of the returning Set<E> should be the same.
+  @override
+  Set<E> toSet() => this;
+
+  /// Returns a copy of other;
+  /// union with an empty set leads to a copy.
+  @override
+  Set<E> union(Set<E> other) => new Set.from(other);
+
+  /// Returns an empty set;
+  /// there are no elements in this (empty) set that are also in other.
+  @override
+  Set<E> intersection(Set<Object> other) => new Set();
+
+  /// Returns an empty set;
+  /// there are no elements in this (empty) set that aren't in other.
+  @override
+  Set<E> difference(Set<Object> other) => new Set();
 }
 
+
 /// Mixin class that implements a throwing version of all set operations that
 /// change the Set.
 abstract class UnmodifiableSetMixin<E> implements Set<E> {

pkgs/collection/test/unmodifiable_collection_test.dart

@@ -35,6 +35,8 @@ main() {
 
   Set aSet = new Set();
   testUnmodifiableSet(aSet, new UnmodifiableSetView(aSet), "empty");
+  aSet = new Set();
+  testUnmodifiableSet(aSet, const UnmodifiableSetView.empty(), "const empty");
   aSet = new Set.from([42]);
   testUnmodifiableSet(aSet, new UnmodifiableSetView(aSet), "single-42");
   aSet = new Set.from([7]);