Pass in "mode" to transformer plugins.

pkg/barback/test/barback_mode_test.dart

+// Copyright (c) 2013, 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 barback.test.barback_mode_test;
+
+import 'package:barback/barback.dart';
+import 'package:unittest/unittest.dart';
+
+import 'utils.dart';
+
+main() {
+  initConfig();
+  test("fromString() uses canonical instances for DEBUG and RELEASE", () {
+    expect(identical(BarbackMode.DEBUG,
+        new BarbackMode.fromString(BarbackMode.DEBUG.name)), isTrue);
+    expect(identical(BarbackMode.RELEASE,
+        new BarbackMode.fromString(BarbackMode.RELEASE.name)), isTrue);
+  });
+}