Add file watching to pub serve.

sdk/lib/_internal/pub/test/serve/watch_added_file_test.dart

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.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.
+
+library pub_tests;
+
+import 'dart:async';
+import 'dart:io';
+
+import 'package:scheduled_test/scheduled_test.dart';
+
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import 'utils.dart';
+
+main() {
+  initConfig();
+  integration("picks up files added after serving started", () {
+    d.dir(appPath, [
+      d.appPubspec(),
+      d.dir("web", [
+        d.file("index.html", "body")
+      ])
+    ]).create();
+
+    startPubServe();
+    waitForBuildSuccess();
+    requestShouldSucceed("index.html", "body");
+
+    d.dir(appPath, [
+      d.dir("web", [
+        d.file("other.html", "added")
+      ])
+    ]).create();
+
+    waitForBuildSuccess();
+    requestShouldSucceed("other.html", "added");
+    endPubServe();
+  });
+}