Make observable transform a barback transform.

pkg/barback/lib/src/transform_logger.dart

Index: pkg/barback/lib/src/transform_logger.dart
diff --git a/pkg/barback/lib/src/transform_logger.dart b/pkg/barback/lib/src/transform_logger.dart
new file mode 100644
index 0000000000000000000000000000000000000000..131be29e8f54d14f39e71378a23c942378ebf443
--- /dev/null
+++ b/pkg/barback/lib/src/transform_logger.dart
@@ -0,0 +1,40 @@
+// 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.transform_logger;
+
+import 'package:source_maps/span.dart';
+
+/// Object used to report warnings and errors encountered while running a
+/// transformer.
+class TransformLogger {
+
+  bool _shouldPrint;
+
+  TransformLogger(this._shouldPrint);
+
+  /// Logs a warning message.
+  ///
+  /// If present, [span] indicates the location in the input asset that caused
+  /// the warning.
+  void warning(String message, [Span span]) {
+    _printMessage('warning', message, span);
+  }
+
+  /// Logs an error message.
+  ///
+  /// If present, [span] indicates the location in the input asset that caused
+  /// the error.
+  // TODO(sigmund,nweiz): clarify when an error should be logged or thrown.
+  void error(String message, [Span span]) {
+    _printMessage('error', message, span);
+  }
+
+  // TODO(sigmund,rnystrom): do something better than printing.
+  _printMessage(String prefix, String message, Span span) {
+    if (!_shouldPrint) return;
+    print(span == null ? '$prefix $message'
+        : '$prefix ${span.getLocationMessage(message)}');
+  }
+}