Add Iterable.empty constructor.

sdk/lib/internal/iterable.dart

Index: sdk/lib/internal/iterable.dart
diff --git a/sdk/lib/internal/iterable.dart b/sdk/lib/internal/iterable.dart
index 0a59b0ea258941304f0e1a89e51b4e8d71eef940..ff03d1a0bb2663b6795e990722c47269ea9dc119 100644
--- a/sdk/lib/internal/iterable.dart
+++ b/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();