Add Iterable.fold (and Stream.fold) which replace `reduce`.

runtime/lib/growable_array.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.
 
 class _GrowableObjectArray<T> implements List<T> {
   factory _GrowableObjectArray._uninstantiable() {
     throw new UnsupportedError(
         "GrowableObjectArray can only be allocated by the VM");
   }
 
@@ skipping 250 matching lines @@
   }
 
   Iterable map(f(T element)) {
     return IterableMixinWorkaround.mapList(this, f);
   }
 
   reduce(initialValue, combine(previousValue, T element)) {
     return IterableMixinWorkaround.reduce(this, initialValue, combine);
   }
 
+  fold(initialValue, combine(previousValue, T element)) {
+    return IterableMixinWorkaround.fold(this, initialValue, combine);
+  }
+
   Iterable<T> where(bool f(T element)) {
     return IterableMixinWorkaround.where(this, f);
   }
 
   Iterable expand(Iterable f(T element)) {
     return IterableMixinWorkaround.expand(this, f);
   }
 
   Iterable<T> take(int n) {
     return IterableMixinWorkaround.takeList(this, n);
@@ skipping 62 matching lines @@
   }
 
   Set<T> toSet() {
     return new Set<T>.from(this);
   }
 
   Map<int, T> asMap() {
     return IterableMixinWorkaround.asMapList(this);
   }
 }