First stab at a dev server in pub using barback.

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

Index: sdk/lib/_internal/pub/test/serve/serve_from_dependency_asset_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/serve_from_dependency_asset_test.dart b/sdk/lib/_internal/pub/test/serve/serve_from_dependency_asset_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..f30d72c353b89b45c6cc3d9499eb422abef240c4
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+++ b/sdk/lib/_internal/pub/test/serve/serve_from_dependency_asset_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("'assets' URLs look in the dependency's asset directory", () {
+    d.dir("foo", [
+      d.libPubspec("foo", "0.0.1"),
+      d.dir("asset", [
+        d.file("foo.txt", "foo"),
+        d.dir("sub", [
+          d.file("bar.txt", "bar"),
+        ])
+      ])
+    ]).create();
+
+    d.dir(appPath, [
+      d.appPubspec({
+        "foo": {"path": "../foo"}
+      })
+    ]).create();
+
+    pubInstall();
+    startPubServe();
+    requestShouldSucceed("assets/foo/foo.txt", "foo");
+    requestShouldSucceed("assets/foo/sub/bar.txt", "bar");
+
+    // "assets" can be in a subpath of the URL:
+    requestShouldSucceed("foo/assets/foo/foo.txt", "foo");
+    requestShouldSucceed("a/b/assets/foo/sub/bar.txt", "bar");
+    endPubServe();
+  });
+}