Add Iterable.empty constructor.

Convenience constructor giving an easy/quick way to create an empty iterable.

R=sgjesse@google.com

Review URL: https://codereview.chromium.org//1152483002

sdk/lib/core/iterable.dart

@@ -101,6 +101,17 @@ abstract class Iterable<E> {
     return new _GeneratorIterable<E>(count, generator);
   }
 
+  /**
+   * Creates an empty iterable.
+   *
+   * The empty iterable has no elements, and iterating it always stops
+   * immediately.
+   *
+   * An empty iterable can be used in places where you always that
+   * the iterable you would otherwise create is empty.
+   */
+  const factory Iterable.empty() = EmptyIterable<E>;
+
   /**
    * Returns a new `Iterator` that allows iterating the elements of this
    * `Iterable`.

sdk/lib/internal/iterable.dart

@@ -658,7 +658,7 @@ class SkipWhileIterator<E> extends Iterator<E> {
  * The always empty [Iterable].
  */
 class EmptyIterable<E> extends Iterable<E>
-                      implements EfficientLengthIterable<E> {
+                       implements EfficientLengthIterable<E> {
   const EmptyIterable();
 
   Iterator<E> get iterator => const EmptyIterator();

tests/corelib/iterable_empty_test.dart

@@ -0,0 +1,72 @@
+// Copyright (c) 2013, 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.
+
+import "package:expect/expect.dart";
+
+main() {
+  testEmpty(name, it, [depth = 2]) {
+    Expect.isTrue(it.isEmpty, name);
+    Expect.isFalse(it.isNotEmpty, name);
+    Expect.equals(0, it.length, name);
+    Expect.isFalse(it.contains(null), name);
+    Expect.isFalse(it.any((x)=>true), name);
+    Expect.isTrue(it.every((x)=>false), name);
+    Expect.throws(() => it.first, (e) => e is StateError, name);
+    Expect.throws(() => it.last, (e) => e is StateError, name);
+    Expect.throws(() => it.single, (e) => e is StateError, name);
+    Expect.throws(() => it.elementAt(0), (e) => e is RangeError, name);
+    Expect.throws(() => it.reduce((a, b) => a), (e) => e is StateError, name);
+    Expect.throws(() => it.singleWhere((_) => true),
+                  (e) => e is StateError, name);
+    Expect.equals(42, it.fold(42, (a, b) => "not 42"), name);
+    Expect.equals(42, it.firstWhere((v) => true, orElse: () => 42), name);
+    Expect.equals(42, it.lastWhere((v) => true, orElse: () => 42), name);
+    Expect.equals("", it.join("separator"), name);
+    Expect.equals("()", it.toString(), name);
+    Expect.listEquals([], it.toList(), name);
+    Expect.listEquals([], it.toList(growable: false), name);
+    Expect.listEquals([], it.toList(growable: true), name);
+    Expect.equals(0, it.toSet().length, name);
+    // Doesn't throw:
+    it.forEach((v) => throw v);
+    for (var v in it) {
+      throw v;
+    }
+    // Check that returned iterables are also empty.
+    if (depth > 0) {
+      testEmpty("$name-map", it.map((x)=>x), depth - 1);
+      testEmpty("$name-where", it.where((x)=>true), depth - 1);
+      testEmpty("$name-expand", it.expand((x)=>[x]), depth - 1);
+      testEmpty("$name-skip", it.skip(1), depth - 1);
+      testEmpty("$name-take", it.take(2), depth - 1);
+      testEmpty("$name-skipWhile", it.skipWhile((v) => false), depth - 1);
+      testEmpty("$name-takeWhile", it.takeWhile((v) => true), depth - 1);
+    }
+  }
+
+  testType(name, it, [depth = 2]) {
+    Expect.isTrue(it is Iterable<int>, name);
+    Expect.isFalse(it is Iterable<String>, name);
+    if (depth > 0) {
+      testType("$name-where", it.where((_)=>true), depth - 1);
+      testType("$name-skip", it.skip(1), depth - 1);
+      testType("$name-take", it.take(1), depth - 1);
+      testType("$name-skipWhile", it.skipWhile((_)=>false), depth - 1);
+      testType("$name-takeWhile", it.takeWhile((_)=>true), depth - 1);
+      testType("$name-toList", it.toList(), depth - 1);
+      testType("$name-toList", it.toList(growable: false), depth - 1);
+      testType("$name-toList", it.toList(growable: true), depth - 1);
+      testType("$name-toSet", it.toSet(), depth - 1);
+    }
+  }
+
+  test(name, it) {
+    testEmpty(name, it);
+    testType(name, it);
+  }
+
+  test("const", const Iterable<int>.empty());
+  test("new", new Iterable<int>.empty());
+}
+