Add onClosed callbacks to HTTP output streams

tests/standalone/io/http_stream_close_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:io";
+import "dart:uri";
+
+main() {
+  bool serverOnClosed = false;
+  bool clientOnClosed = false;
+
+  var server = new HttpServer();
+  server.listen("127.0.0.1", 0);
+  server.defaultRequestHandler = (request, response) {
+    request.inputStream.onData = request.inputStream.read;
+    request.inputStream.onClosed = () {
+      response.outputStream.onClosed = () => serverOnClosed = true;
+      response.outputStream.writeString("hello!");
+      response.outputStream.close();
+    };
+  };
+
+  var client = new HttpClient();
+    var connection = client.postUrl(new Uri.fromString("http://127.0.0.1:${server.port}"));
+    connection.onError = (e) => throw e;
+    connection.onRequest = (request) {
+      request.contentLength = "hello!".length;
+      request.outputStream.onError = (e) => throw e;
+      request.outputStream.onClosed = () => clientOnClosed = true;
+      request.outputStream.writeString("hello!");
+      request.outputStream.close();
+    };
+    connection.onResponse = (response) {
+      response.inputStream.onData = response.inputStream.read;
+      response.inputStream.onClosed = () {
+        Expect.isTrue(serverOnClosed && clientOnClosed);
+        server.close();
+        client.shutdown();
+      };
+    };
+}