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

tests/html/streams_test.dart

 library streams_test;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:async';
 import 'dart:html';
 
 class StreamHelper {
   var _a;
   StreamHelper() {
     _a = new TextInputElement();
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   });
 
   test('map', () {
     stream.map((_) => null).listen((_) {});
   });
 
   test('reduce', () {
     stream.reduce(null, (a, b) => null).then((_) {});
   });
 
+  test('fold', () {
+    stream.fold(null, (a, b) => null).then((_) {});
+  });
+
   test('contains', () {
     stream.contains((_) => true).then((_) {});
   });
 
   test('every', () {
     stream.every((_) => true).then((_) {});
   });
 
   test('any', () {
     stream.any((_) => true).then((_) {});
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   });
 
   test('singleWhere', () {
     stream.singleWhere((_) => true).then((_) {});
   });
 
   test('elementAt', () {
     stream.elementAt(0).then((_) {});
   });
 }