Add a Futures.forEach function for asynchronously iterating over a collection.

sdk/lib/core/future.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.
 
 /**
  * A [Future] is used to obtain a value sometime in the future.  Receivers of a
  * [Future] can obtain the value by passing a callback to [then]. For example:
  *
  *     Future<int> future = getFutureFromSomewhere();
  *     future.then((value) {
@@ skipping 241 matching lines @@
       });
       future.handleException((exception) {
         if (!result.isComplete) {
           completer.completeException(exception, future.stackTrace);
         }
         return true;
       });
     }
     return result;
   }
+
+  /**
+   * Runs [f] for each element in [input] in order, moving to the next element
+   * only when the [Future] returned by [f] completes. Returns a [Future] that
+   * completes when all elements have been processed.
+   *
+   * The return values of all [Future]s are discarded. Any errors will cause the
+   * iteration to stop and will be piped through the returned [Future].
+   */
+  static Future forEach(Iterable input, Future f(element)) {
+    var iterator = input.iterator();
+    Future nextElement(_) {
+      if (!iterator.hasNext) return new Future.immediate(null);
+      return f(iterator.next()).chain(nextElement);
+    }
+    return nextElement(null);
+  }
 }