Found some more code referring to the special "asset" directory.

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 '../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();
 
     pubGet();
     pubServe();
     requestShould404("packages/foo/nope.dart");
-    requestShould404("assets/foo/nope.png");
     requestShould404("dir/packages/foo/nope.dart");
-    requestShould404("dir/assets/foo/nope.png");
     endPubServe();
   });
 }