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

tests/corelib/futures_test.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.
 
 library futures_test;
 import 'dart:isolate';
 
 Future testWaitEmpty() {
   List<Future> futures = new List<Future>();
   return Futures.wait(futures);
 }
 
 Future testCompleteAfterWait() {
   List<Future> futures = new List<Future>();
   Completer<Object> c = new Completer<Object>();
   futures.add(c.future);
   Future future = Futures.wait(futures);
   c.complete(null);
   return future;
 }
 
 Future testCompleteBeforeWait() {
   List<Future> futures = new List<Future>();
   Completer c = new Completer();
   futures.add(c.future);
   c.complete(null);
   return Futures.wait(futures);
 }
 
+Future testForEachEmpty() {
+  return Futures.forEach([], (_) => throw 'should not be called');
+}
+
+Future testForEach() {
+  var seen = <int>[];
+  return Futures.forEach([1, 2, 3, 4, 5], (n) {
+    seen.add(n);
+    return new Future.immediate(null);
+  }).transform((_) => Expect.listEquals([1, 2, 3, 4, 5], seen));
+}
+
+Future testForEachWithException() {
+  var seen = <int>[];
+  return Futures.forEach([1, 2, 3, 4, 5], (n) {
+    if (n == 4) throw 'correct exception';
+    seen.add(n);
+    return new Future.immediate(null);
+  }).transform((_) => throw 'incorrect exception').transformException((e) {
+    Expect.equals('correct exception', e);
+  });
+}
+
 main() {
   List<Future> futures = new List<Future>();
 
   futures.add(testWaitEmpty());
   futures.add(testCompleteAfterWait());
   futures.add(testCompleteBeforeWait());
+  futures.add(testForEachEmpty());
+  futures.add(testForEach());
 
   // Use a receive port for blocking the test. 
   // Note that if the test fails, the program will not end.
   ReceivePort port = new ReceivePort();
   Futures.wait(futures).then((List list) {
-    Expect.equals(3, list.length);
+    Expect.equals(5, list.length);
     port.close();
   });
 }