Support loading transformer plugins from pub.

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

Index: sdk/lib/_internal/pub/test/serve/prints_a_transform_interface_error.dart
diff --git a/sdk/lib/_internal/pub/test/serve/prints_a_transform_interface_error.dart b/sdk/lib/_internal/pub/test/serve/prints_a_transform_interface_error.dart
new file mode 100644
index 0000000000000000000000000000000000000000..11464351a1561dac0a28fbef23f92c27cd5b5dcb
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+++ b/sdk/lib/_internal/pub/test/serve/prints_a_transform_interface_error.dart
@@ -0,0 +1,47 @@
+// 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 '../descriptor.dart' as d;
+import '../test_pub.dart';
+import 'utils.dart';
+
+main() {
+  initConfig();
+  integration("prints a transform interface error", () {
+    var transformer = """
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+class RewriteTransformer extends Transformer {
+  RewriteTransformer();
+
+  String get allowedExtensions => '.txt';
+}
+""";
+
+    d.dir(appPath, [
+      d.pubspec({
+        "name": "myapp",
+        "transformers": ["myapp/src/transformer"]
+      }),
+      d.dir("lib", [d.dir("src", [
+        d.file("transformer.dart", transformer)
+      ])]),
+      d.dir("web", [
+        d.file("foo.txt", "foo")
+      ])
+    ]).create();
+
+    createLockFile('myapp', {}, pkg: ['barback']);
+
+    var server = startPubServe();
+    expect(server.nextLine(), completion(equals('Build error:')));
+    expect(server.nextLine(), completion(equals("| Class 'RewriteTransformer' "
+        "has no instance method 'apply'.")));
+    endPubServe();
+  });
+}