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

tests/standalone/io/string_decoder_test.dart

 // Copyright (c) 2013, 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.
 
 import "dart:async";
 import "dart:io";
 
 void test() {
   // Code point U+10FFFF is the largest code point supported by Dart.
   var controller = new StreamController();
   controller.add([0xf0, 0x90, 0x80, 0x80]);  // U+10000
   controller.add([0xf4, 0x8f, 0xbf, 0xbf]);  // U+10FFFF
   controller.add([0xf4, 0x90, 0x80, 0x80]);  // U+110000
   controller.add([0xfa, 0x80, 0x80, 0x80, 0x80]);  //  U+2000000
   controller.add([0xfd, 0x80, 0x80, 0x80, 0x80, 0x80]);  // U+40000000
   controller.close();
 
   var decoder = new StringDecoder(Encoding.UTF_8, '?'.codeUnitAt(0));
   var stream = controller.stream.transform(decoder);
-  stream.reduce(
+  stream.fold(
       new StringBuffer(),
       (b, e) {
         b.write(e);
         return b;
       })
       .then((b) => b.toString())
       .then((decoded) {
         Expect.equals(7, decoded.length);
 
         var replacementChar = '?'.codeUnitAt(0);
@@ skipping 24 matching lines @@
   invalid([0xff], 1);
   invalid([0xf0, 0xc0], 1);
   invalid([0xc0, 0x80], 1);
   invalid([0xfd, 0x80, 0x80], 3); // Unfinished encoding.
 }
 
 void main() {
   test();
   testInvalid();
 }