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

tests/standalone/io/secure_multiple_client_server_test.dart

Index: tests/standalone/io/secure_multiple_client_server_test.dart
diff --git a/tests/standalone/io/secure_multiple_client_server_test.dart b/tests/standalone/io/secure_multiple_client_server_test.dart
index 27527b15ff2eb6d99de7fcb909302e0e7d0f525e..4700fcd45d2b094d7fe598d81e7e2681ee397cf3 100644
--- a/tests/standalone/io/secure_multiple_client_server_test.dart
+++ b/tests/standalone/io/secure_multiple_client_server_test.dart
@@ -20,7 +20,7 @@ Future<SecureServerSocket> startServer() {
                                  5,
                                  CERTIFICATE).then((server) {
     server.listen((SecureSocket client) {
-      client.reduce(<int>[], (message, data) => message..addAll(data))
+      client.fold(<int>[], (message, data) => message..addAll(data))
           .then((message) {
             String received = new String.fromCharCodes(message);
             Expect.isTrue(received.contains("Hello from client "));
@@ -37,7 +37,7 @@ Future testClient(server, name) {
   return SecureSocket.connect(HOST_NAME, server.port).then((socket) {
     socket.writeBytes("Hello from client $name".codeUnits);
     socket.close();
-    return socket.reduce(<int>[], (message, data) => message..addAll(data))
+    return socket.fold(<int>[], (message, data) => message..addAll(data))
         .then((message) {
           Expect.listEquals("Welcome, client $name".codeUnits, message);
           return server;