Pass logs through transformer groups.

pkg/barback/test/transformer/log.dart

Index: pkg/barback/test/transformer/log.dart
diff --git a/pkg/barback/test/transformer/log.dart b/pkg/barback/test/transformer/log.dart
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+// 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.transformer.log;
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+import 'package:barback/src/utils.dart';
+
+import 'mock.dart';
+
+/// A transformer that logs given entries during its apply.
+class LogTransformer extends MockTransformer {
+  /// The list of entries that it should log.
+  ///
+  /// Each entry has the log level followed by the message, like:
+  ///
+  ///     error: This is the error message.
+  final List<String> _entries;
+
+  LogTransformer(this._entries);
+
+  Future<bool> doIsPrimary(Asset asset) => new Future.value(true);
+
+  Future doApply(Transform transform) {
+    return newFuture(() {
+      for (var entry in _entries) {
+        var parts = entry.split(":");
+        var logFn;
+        switch (parts[0]) {
+          case "error":   logFn = transform.logger.error; break;
+          case "warning": logFn = transform.logger.warning; break;
+          case "info":    logFn = transform.logger.info; break;
+        }
+
+        logFn(parts[1].trim());
+      }
+    });
+  }
+}