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

sdk/lib/_internal/pub/test/dart2js/ignores_entrypoints_in_lib_and_asset_test.dart

Index: sdk/lib/_internal/pub/test/dart2js/ignores_entrypoints_in_lib_and_asset_test.dart
diff --git a/sdk/lib/_internal/pub/test/dart2js/ignores_entrypoints_in_lib_and_asset_test.dart b/sdk/lib/_internal/pub/test/dart2js/ignores_entrypoints_in_lib_and_asset_test.dart
deleted file mode 100644
index e09dd7f01ac1a53643567118073e8aa292e03ff1..0000000000000000000000000000000000000000
--- a/sdk/lib/_internal/pub/test/dart2js/ignores_entrypoints_in_lib_and_asset_test.dart
+++ /dev/null
@@ -1,47 +0,0 @@
-// 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.
-
-import 'package:scheduled_test/scheduled_test.dart';
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-import '../serve/utils.dart';
-
-main() {
-  initConfig();
-  setUp(() {
-    d.dir(appPath, [
-      d.appPubspec(),
-      d.dir('asset', [
-        d.file('file.dart', 'void main() => print("hello");'),
-      ]),
-      d.dir('lib', [
-        d.file('file.dart', 'void main() => print("hello");'),
-      ]),
-      d.dir('web', [
-        d.file('index.html', 'html'),
-      ])
-    ]).create();
-  });
-
-  integration("build ignores Dart entrypoints in lib and asset", () {
-    schedulePub(args: ["build", "--all"],
-        output: new RegExp(r'Built 1 file to "build".'));
-
-    d.dir(appPath, [
-      d.dir('build', [
-        d.nothing('asset'),
-        d.nothing('lib')
-      ])
-    ]).validate();
-  });
-
-  integration("serve ignores Dart entrypoints in lib and asset", () {
-    pubServe();
-    requestShould404("assets/myapp/main.dart.js");
-    requestShould404("packages/myapp/main.dart.js");
-    endPubServe();
-  });
-
-}