Added expand method to iterables.

sdk/lib/collection/collections.dart

 // Copyright (c) 2012, 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.collection;
 
 /**
  * This class provides default implementations for Iterables (including Lists).
  *
  * Once Dart receives Mixins it will be replaced with mixin classes.
@@ skipping 308 matching lines @@
 
   static Iterable mapList(List list, f(var element)) {
     return new MappedListIterable(list, f, 0, null);
   }
 
   static List mappedByList(List list, f(var element)) {
     // This is currently a List as well as an Iterable.
     return new MappedList(list, f);
   }
 
+  static Iterable expand(Iterable iterable, Iterable f(var element)) {
+    return new ExpandIterable(iterable, f);
+  }
+
   static Iterable takeList(List list, int n) {
     // The generic type is currently lost. It will be fixed with mixins.
     // This is currently a List as well as an Iterable.
     return new ListView(list, 0, n);
   }
 
   static Iterable takeWhile(Iterable iterable, bool test(var value)) {
     // The generic type is currently lost. It will be fixed with mixins.
     return new TakeWhileIterable(iterable, test);
   }
@@ skipping 126 matching lines @@
   static Iterable skipList(List list, int n)
       => IterableMixinWorkaround.skipList(list, n);
 
   /** Deprecated. Use the same method in [IterableMixinWorkaround] instead.*/
   static Iterable skipWhile(Iterable iterable, bool test(var value))
       => IterableMixinWorkaround.skipWhile(iterable, test);
 
   static String collectionToString(Collection c)
       => ToString.collectionToString(c);
 }