First stab at a dev server in pub using barback.

sdk/lib/_internal/pub/test/serve/missing_dependency_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 a missing files in dependencies", () {
+    d.dir("foo", [
+      d.libPubspec("foo", "0.0.1")
+    ]).create();
+
+    d.dir(appPath, [
+      d.appPubspec({
+        "foo": {"path": "../foo"}
+      })
+    ]).create();
+
+    pubInstall();
+    startPubServe();
+    requestShould404("packages/foo/nope.dart");
+    requestShould404("assets/foo/nope.png");
+    requestShould404("dir/packages/foo/nope.dart");
+    requestShould404("dir/assets/foo/nope.png");
+    endPubServe();
+  });
+}