refactor(dart/transform): Minor logging changes

Enable easier testing by providing a null log implementation and a way
to use it.

modules/angular2/test/transform/common/logger.dart

@@ -0,0 +1,23 @@
+library angular2.test.transform.common.logger;
+
+import 'package:code_transformers/messages/build_logger.dart';
+
+class NullLogger implements BuildLogger {
+  const NullLogger();
+  void info(String message, {AssetId asset, SourceSpan span}) {}
+  void fine(String message, {AssetId asset, SourceSpan span}) {}
+  void warning(String message, {AssetId asset, SourceSpan span}) {}
+  void error(String message, {AssetId asset, SourceSpan span}) {
+    throw new NullLoggerError(message, asset, span);
+  }
+  Future writeOutput() => null;
+  Future addLogFilesFromAsset(AssetId id, [int nextNumber = 1]) => null;
+}
+
+class NullLoggerError extends Error {
+  final String message;
+  final AssetId asset;
+  final SourceSpan span;
+
+  NullLoggerError(message, asset, span);
+}