Add file watching to pub serve.

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

Index: sdk/lib/_internal/pub/test/serve/watch_modified_file_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/missing_asset_test.dart b/sdk/lib/_internal/pub/test/serve/watch_modified_file_test.dart
similarity index 54%
copy from sdk/lib/_internal/pub/test/serve/missing_asset_test.dart
copy to sdk/lib/_internal/pub/test/serve/watch_modified_file_test.dart
index 1f69b6c4471db387328852e83ff03d6865ad829b..864d8aa1c943f0a4cbe2482c7f12d32d90dcc908 100644
--- a/sdk/lib/_internal/pub/test/serve/missing_asset_test.dart
+++ b/sdk/lib/_internal/pub/test/serve/watch_modified_file_test.dart
@@ -12,17 +12,25 @@ 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();
   });
 }