First stab at a dev server in pub using barback.

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

Index: sdk/lib/_internal/pub/test/serve/missing_file_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/missing_file_test.dart b/sdk/lib/_internal/pub/test/serve/missing_file_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..cfd7d07770f0a897041b63585fb774ef63a6f2e8
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+++ b/sdk/lib/_internal/pub/test/serve/missing_file_test.dart
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+// 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 files", () {
+    d.dir(appPath, [
+      d.appPubspec()
+    ]).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");
+    endPubServe();
+  });
+}