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..616009d2b79b9c7cf044bede4d2410470654d3cd 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 know that
+   * the iterable you would otherwise is always empty.

[Søren Gjesse, 2015/05/21 09:10:06]

Seems like there is a word missing after "otherwise".

+   */
+  const factory Iterable.empty() = EmptyIterable<E>;

[kevmoo, 2015/05/21 15:07:06]

Why is this useful over using `const []`?

[Lasse Reichstein Nielsen, 2015/05/21 15:16:51]

Because const[] is a List, not just an Iterable.
You shouldn't return a class that is more capable than our interface requires,
at least not in library code, because sooner or later someone is going to rely
on it.

I was about to do a new DelegatingIterable(const []) when I decided that I
should just get around to add this constructor (which I had plans for before I
even started working on the libraries :).

And the class already existed, so it was either moving it to a public library
(from dart:_internal) or giving access to it like this.

+
+  /**
    * Returns a new `Iterator` that allows iterating the elements of this
    * `Iterable`.
    *