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

sdk/lib/_internal/pub/test/serve/missing_asset_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 missing assets", () {
     d.dir(appPath, [
       d.appPubspec()
     ]).create();
 
     pubServe();
     requestShould404("index.html");
     requestShould404("packages/myapp/nope.dart");
-    requestShould404("assets/myapp/nope.png");
     requestShould404("dir/packages/myapp/nope.dart");
-    requestShould404("dir/assets/myapp/nope.png");
     endPubServe();
   });
 }