Properly handle dart2js being specified before other local transformers.

sdk/lib/_internal/pub_generated/test/transformer/dart2js_transformer_before_another_transformer_test.dart

Index: sdk/lib/_internal/pub_generated/test/transformer/dart2js_transformer_before_another_transformer_test.dart
diff --git a/sdk/lib/_internal/pub_generated/test/transformer/runs_a_local_transform_on_the_application_package_test.dart b/sdk/lib/_internal/pub_generated/test/transformer/dart2js_transformer_before_another_transformer_test.dart
similarity index 81%
copy from sdk/lib/_internal/pub_generated/test/transformer/runs_a_local_transform_on_the_application_package_test.dart
copy to sdk/lib/_internal/pub_generated/test/transformer/dart2js_transformer_before_another_transformer_test.dart
index 714d8fba04f4598bff62e5dfb8a5dcb6a142a91c..5be8957e92961cb9e03f238ab412fedb6e7a3485 100644
--- a/sdk/lib/_internal/pub_generated/test/transformer/runs_a_local_transform_on_the_application_package_test.dart
+++ b/sdk/lib/_internal/pub_generated/test/transformer/dart2js_transformer_before_another_transformer_test.dart
@@ -8,13 +8,14 @@ import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import '../serve/utils.dart';
 
+// Regression test for issue 21726.
 main() {
   initConfig();
   withBarbackVersions("any", () {
-    integration("runs a local transform on the application package", () {
+    integration("runs a dart2js transformer before a local transformer", () {
       d.dir(appPath, [d.pubspec({
           "name": "myapp",
-          "transformers": ["myapp/src/transformer"]
+          "transformers": [r"$dart2js", "myapp/src/transformer"]
         }),
             d.dir("lib", [d.dir("src", [d.file("transformer.dart", REWRITE_TRANSFORMER)])]),
             d.dir("web", [d.file("foo.txt", "foo")])]).create();