Add Iterable.empty constructor.

sdk/lib/core/iterable.dart

Index: sdk/lib/core/iterable.dart
diff --git a/sdk/lib/core/iterable.dart b/sdk/lib/core/iterable.dart
index e435b1e7680786539216842297b416d9cc6541e1..c22283ab4e16da45383202539aa9494da16857f2 100644
--- a/sdk/lib/core/iterable.dart
+++ b/sdk/lib/core/iterable.dart
@@ -102,6 +102,17 @@ abstract class Iterable<E> {
   }
 
   /**
+   * 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`.
    *