First stab at a dev server in pub using barback.

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

Index: sdk/lib/_internal/pub/test/serve/serve_from_app_lib_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/serve_from_app_lib_test.dart b/sdk/lib/_internal/pub/test/serve/serve_from_app_lib_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..0ddea5fa0695911cc9f3aaa75b656a6935469ebb
--- /dev/null
+++ b/sdk/lib/_internal/pub/test/serve/serve_from_app_lib_test.dart
@@ -0,0 +1,35 @@
+// 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("'packages' URLs look in the app's lib directory", () {
+    d.dir(appPath, [
+      d.appPubspec(),
+      d.dir("lib", [
+        d.file("lib.dart", "foo() => 'foo';"),
+        d.dir("sub", [
+          d.file("lib.dart", "bar() => 'bar';"),
+        ])
+      ])
+    ]).create();
+
+    startPubServe();
+    requestShouldSucceed("packages/myapp/lib.dart", "foo() => 'foo';");
+    requestShouldSucceed("packages/myapp/sub/lib.dart", "bar() => 'bar';");
+
+    // "packages" can be in a subpath of the URL:
+    requestShouldSucceed("foo/packages/myapp/lib.dart", "foo() => 'foo';");
+    requestShouldSucceed("a/b/packages/myapp/sub/lib.dart", "bar() => 'bar';");
+    endPubServe();
+  });
+}