Add Iterable.empty constructor.

sdk/lib/internal/iterable.dart

 // Copyright (c) 2011, 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.
 
 part of dart._internal;
 
 /**
  * An [Iterable] for classes that have efficient [length] and [elementAt].
  *
  * All other methods are implemented in terms of [length] and [elementAt],
@@ skipping 640 matching lines @@
     return _iterator.moveNext();
   }
 
   E get current => _iterator.current;
 }
 
 /**
  * The always empty [Iterable].
  */
 class EmptyIterable<E> extends Iterable<E>
-                      implements EfficientLengthIterable<E> {
+                       implements EfficientLengthIterable<E> {
   const EmptyIterable();
 
   Iterator<E> get iterator => const EmptyIterator();
 
   void forEach(void action(E element)) {}
 
   bool get isEmpty => true;
 
   int get length => 0;
 
@@ skipping 76 matching lines @@
  * Creates errors throw by [Iterable] when the element count is wrong.
  */
 abstract class IterableElementError {
   /** Error thrown thrown by, e.g., [Iterable.first] when there is no result. */
   static StateError noElement() => new StateError("No element");
   /** Error thrown by, e.g., [Iterable.single] if there are too many results. */
   static StateError tooMany() => new StateError("Too many elements");
   /** Error thrown by, e.g., [List.setRange] if there are too few elements. */
   static StateError tooFew() => new StateError("Too few elements");
 }