Make mappedBy lazy.

sdk/lib/async/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 209 matching lines @@
  * [Futures] holds additional utility functions that operate on [Future]s (for
  * example, waiting for a collection of Futures to complete).
  */
 class Futures {
   /**
    * Returns a future which will complete once all the futures in a list are
    * complete. If any of the futures in the list completes with an exception,
    * the resulting future also completes with an exception. (The value of the
    * returned future will be a list of all the values that were produced.)
    */
-  static Future<List> wait(List<Future> futures) {
+  static Future<List> wait(Iterable<Future> futures) {
     if (futures.isEmpty) {
       return new Future<List>.immediate(const []);
     }
 
     Completer completer = new Completer<List>();
     Future<List> result = completer.future;
-    int remaining = futures.length;
-    List values = new List(futures.length);
+    int remaining = 0;
+    int seen = 0;
+    bool initializing = true;
+    List values = [];
 
     // As each future completes, put its value into the corresponding
     // position in the list of values.
-    for (int i = 0; i < futures.length; i++) {
-      // TODO(mattsh) - remove this after bug
-      // http://code.google.com/p/dart/issues/detail?id=333 is fixed.
-      int pos = i;
-      Future future = futures[pos];
+    for (Future future in futures) {
+      int pos = seen;
+      remaining++;
+      seen++;
+      values.length = seen;
       future.then((Object value) {
         values[pos] = value;
-        if (--remaining == 0 && !result.isComplete) {
+        if (--remaining == 0 && !result.isComplete && !initializing) {
           completer.complete(values);
         }
       });
       future.handleException((exception) {
         if (!result.isComplete) {
           completer.completeException(exception, future.stackTrace);
         }
         return true;
       });
     }
+    initializing = false;
+    if (remaining == 0 && !result.isComplete) {
+      // All futures already had their values set.
+      completer.complete(values);
+    }
     return result;
   }
 }