First stab at a dev server in pub using barback.

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

Index: sdk/lib/_internal/pub/test/serve/serve_from_dependency_lib_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/serve_from_dependency_lib_test.dart b/sdk/lib/_internal/pub/test/serve/serve_from_dependency_lib_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..a6a9ab2edc39b25b55fbc30fbc4b91b440abb425
--- /dev/null
+++ b/sdk/lib/_internal/pub/test/serve/serve_from_dependency_lib_test.dart
@@ -0,0 +1,42 @@
+// 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 dependency's lib directory", () {
+    d.dir("foo", [
+      d.libPubspec("foo", "0.0.1"),
+      d.dir("lib", [
+        d.file("lib.dart", "foo() => 'foo';"),
+        d.dir("sub", [
+          d.file("lib.dart", "bar() => 'bar';"),
+        ])
+      ])
+    ]).create();
+
+    d.dir(appPath, [
+      d.appPubspec({
+        "foo": {"path": "../foo"}
+      })
+    ]).create();
+
+    pubInstall();
+    startPubServe();
+    requestShouldSucceed("packages/foo/lib.dart", "foo() => 'foo';");
+    requestShouldSucceed("packages/foo/sub/lib.dart", "bar() => 'bar';");
+
+    // "packages" can be in a subpath of the URL:
+    requestShouldSucceed("foo/packages/foo/lib.dart", "foo() => 'foo';");
+    requestShouldSucceed("a/b/packages/foo/sub/lib.dart", "bar() => 'bar';");
+    endPubServe();
+  });
+}