Don't load built-in transformers for "pub run".

sdk/lib/_internal/pub/test/run/ignores_explicit_dart2js_transformer_test.dart

Index: sdk/lib/_internal/pub/test/run/ignores_explicit_dart2js_transformer_test.dart
diff --git a/sdk/lib/_internal/pub/test/run/nonexistent_script_test.dart b/sdk/lib/_internal/pub/test/run/ignores_explicit_dart2js_transformer_test.dart
similarity index 55%
copy from sdk/lib/_internal/pub/test/run/nonexistent_script_test.dart
copy to sdk/lib/_internal/pub/test/run/ignores_explicit_dart2js_transformer_test.dart
index ce385fc0dd9b66a487aa906055b836674412473c..ffeb779eab7ec07870e2488a154d45a711f40764 100644
--- a/sdk/lib/_internal/pub/test/run/nonexistent_script_test.dart
+++ b/sdk/lib/_internal/pub/test/run/ignores_explicit_dart2js_transformer_test.dart
@@ -2,21 +2,24 @@
 // 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:path/path.dart' as p;
-
-import '../../lib/src/exit_codes.dart' as exit_codes;
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 
 main() {
   initConfig();
-  integration('Errors if the script does not exist.', () {
+  integration("doesn't choke on an explicit dart2js transformer", () {
     d.dir(appPath, [
-      d.appPubspec()
+      d.pubspec({
+        "name": "myapp",
+        "transformers": [r"$dart2js"]
+      }),
+      d.dir("bin", [
+        d.file("script.dart", "main() => print('Hello!');")
+      ])
     ]).create();
 
     var pub = pubRun(args: ["script"]);
-    pub.stderr.expect("Could not find ${p.join("bin", "script.dart")}.");
-    pub.shouldExit(exit_codes.NO_INPUT);
+    pub.stdout.expect("Hello!");
+    pub.shouldExit(0);
   });
 }