Support loading transformer plugins from pub.

sdk/lib/_internal/pub/test/serve/fails_to_load_a_non_existent_transform_test.dart

Index: sdk/lib/_internal/pub/test/serve/fails_to_load_a_non_existent_transform_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/fails_to_load_a_non_existent_transform_test.dart b/sdk/lib/_internal/pub/test/serve/fails_to_load_a_non_existent_transform_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..31e56aa7c22233e5b6228685923e25fd8d299304
--- /dev/null
+++ b/sdk/lib/_internal/pub/test/serve/fails_to_load_a_non_existent_transform_test.dart
@@ -0,0 +1,27 @@
+// 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 'package:scheduled_test/scheduled_test.dart';
+
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+
+main() {
+  initConfig();
+  integration("fails to load a non-existent transform", () {
+    d.dir(appPath, [
+      d.pubspec({
+        "name": "myapp",
+        "transformers": ["myapp/transform"]
+      })
+    ]).create();
+
+    var pub = startPub(args: ['serve', '--port=0']);
+    expect(pub.nextErrLine(), completion(equals(
+        'Transformer library "package:myapp/transform.dart" not found.')));
+    pub.shouldExit(1);
+  });
+}