Improve parallelism when loading transformer plugins.

sdk/lib/_internal/pub/test/transformers_needed_by_transformers/error_test.dart

Index: sdk/lib/_internal/pub/test/transformers_needed_by_transformers/error_test.dart
diff --git a/sdk/lib/_internal/pub/test/transformers_needed_by_transformers/error_test.dart b/sdk/lib/_internal/pub/test/transformers_needed_by_transformers/error_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..0908cb4f917df710e3c807a852933798f771bae6
--- /dev/null
+++ b/sdk/lib/_internal/pub/test/transformers_needed_by_transformers/error_test.dart
@@ -0,0 +1,49 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS 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';
+import 'utils.dart';
+
+void main() {
+  initConfig();
+
+  integration("fails if an unknown package is imported", () {
+    d.dir(appPath, [
+      d.pubspec({
+        "name": "myapp",
+        "transformers": ["myapp"]
+      }),
+      d.dir('lib', [
+        d.file("myapp.dart", transformer(["package:foo/foo.dart"]))
+      ])
+    ]).create();
+
+    expectException(predicate((error) {
+      expect(error, new isInstanceOf<ApplicationException>());
+      expect(error.message, equals(
+          'A transformer imported unknown package "foo" (in '
+          '"package:foo/foo.dart").'));
+      return true;
+    }));
+  });
+
+  integration("fails on a syntax error", () {
+    d.dir(appPath, [
+      d.pubspec({
+        "name": "myapp",
+        "transformers": ["myapp"]
+      }),
+      d.dir('lib', [
+        d.file("myapp.dart", "library;")
+      ])
+    ]).create();
+
+    expectException(new isInstanceOf<AnalyzerErrorGroup>());
+  });
+}