Add file watching to pub serve.

sdk/lib/_internal/pub/test/serve/watch_modified_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:io';
 
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import 'utils.dart';
 
 main() {
   initConfig();
-  integration("responds with a 404 for missing assets", () {
+  integration("watches modifications to files", () {
     d.dir(appPath, [
-      d.appPubspec()
+      d.appPubspec(),
+      d.dir("web", [
+        d.file("index.html", "before")
+      ])
     ]).create();
 
     startPubServe();
-    requestShould404("index.html");
-    requestShould404("packages/myapp/nope.dart");
-    requestShould404("assets/myapp/nope.png");
-    requestShould404("dir/packages/myapp/nope.dart");
-    requestShould404("dir/assets/myapp/nope.png");
+    requestShouldSucceed("index.html", "before");
+
+    d.dir(appPath, [
+      d.dir("web", [
+        d.file("index.html", "after")
+      ])
+    ]).create();
+
+    waitForBuildSuccess();
+    requestShouldSucceed("index.html", "after");
     endPubServe();
   });
 }