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

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

Index: sdk/lib/_internal/pub/test/dart2js/ignores_entrypoints_in_lib_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_test.dart
similarity index 74%
rename from sdk/lib/_internal/pub/test/dart2js/ignores_entrypoints_in_lib_and_asset_test.dart
rename to sdk/lib/_internal/pub/test/dart2js/ignores_entrypoints_in_lib_test.dart
index e09dd7f01ac1a53643567118073e8aa292e03ff1..73dda873879b555896218137e5131486b4f918d9 100644
--- 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_test.dart
@@ -13,9 +13,6 @@ main() {
   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");'),
       ]),
@@ -25,21 +22,19 @@ main() {
     ]).create();
   });
 
-  integration("build ignores Dart entrypoints in lib and asset", () {
+  integration("build ignores Dart entrypoints in lib", () {
     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", () {
+  integration("serve ignores Dart entrypoints in lib", () {
     pubServe();
-    requestShould404("assets/myapp/main.dart.js");
     requestShould404("packages/myapp/main.dart.js");
     endPubServe();
   });