Add file watching to pub serve.

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

Index: sdk/lib/_internal/pub/test/serve/watch_removed_file_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/missing_asset_test.dart b/sdk/lib/_internal/pub/test/serve/watch_removed_file_test.dart
similarity index 52%
copy from sdk/lib/_internal/pub/test/serve/missing_asset_test.dart
copy to sdk/lib/_internal/pub/test/serve/watch_removed_file_test.dart
index 1f69b6c4471db387328852e83ff03d6865ad829b..f2a41351618212582cb755b9de09d6a3a908ef8e 100644
--- a/sdk/lib/_internal/pub/test/serve/missing_asset_test.dart
+++ b/sdk/lib/_internal/pub/test/serve/watch_removed_file_test.dart
@@ -6,23 +6,32 @@ library pub_tests;
 
 import 'dart:io';
 
+import 'package:path/path.dart' as path;
+import 'package:scheduled_test/scheduled_test.dart';
+
+import '../../lib/src/io.dart';
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import 'utils.dart';
 
 main() {
   initConfig();
-  integration("responds with a 404 for missing assets", () {
+  integration("stop serving a file that is removed", () {
     d.dir(appPath, [
-      d.appPubspec()
+      d.appPubspec(),
+      d.dir("web", [
+        d.file("index.html", "body")
+      ])
     ]).create();
 
     startPubServe();
+    requestShouldSucceed("index.html", "body");
+
+    schedule(() => deleteEntry(
+        path.join(sandboxDir, appPath, "web", "index.html")));
+
+    waitForBuildSuccess();
     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");
     endPubServe();
   });
 }