Support loading transformer plugins from pub.

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

Index: sdk/lib/_internal/pub/test/serve/prints_a_transform_error_in_apply.dart
diff --git a/sdk/lib/_internal/pub/test/serve/prints_a_transform_error_in_apply.dart b/sdk/lib/_internal/pub/test/serve/prints_a_transform_error_in_apply.dart
new file mode 100644
index 0000000000000000000000000000000000000000..5988f7b675b6cc0ea7f8585869dbc588ca248325
--- /dev/null
+++ b/sdk/lib/_internal/pub/test/serve/prints_a_transform_error_in_apply.dart
@@ -0,0 +1,50 @@
+// 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 error in apply", () {
+    var transformer = """

[Bob Nystrom, 2013/08/27 22:12:30]

Can you move this to the top-level just so that the lack of indentation doesn't
make the code harder to read?

[nweiz, 2013/08/28 20:45:23]

Done.

+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+class RewriteTransformer extends Transformer {
+  RewriteTransformer();
+
+  String get allowedExtensions => '.txt';
+
+  Future apply(Transform transform) => throw new Exception('oh no!');
+}
+""";
+
+    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:')));

[Bob Nystrom, 2013/08/27 22:12:30]

Would be great to include the name of the transformer and primary input here.
Maybe do that now, or add a TODO and file a bug?

[nweiz, 2013/08/28 20:45:23]

It turns out this test wasn't actually getting run, and the real output does
include the transform information. Fixed.

+    expect(server.nextLine(), completion(equals('| oh no!')));
+    expect(server.nextLine(), completion(isEmpty));
+    expect(server.nextLine(), completion(equals('Stack trace:')));
+    endPubServe();
+  });
+}