refactor(dart/transform): Minor logging changes

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

modules/angular2/src/transform/common/logging.dart

@@ -10,6 +10,11 @@ void init(Transform t) {
   _logger = new BuildLogger(t);
 }
 
+/// Sets [logger] directly. Used for testing - in general use [init].
+void setLogger(BuildLogger logger) {
+  _logger = logger;
+}
+
 /// The logger the transformer should use for messaging.
 BuildLogger get logger {
   if (_logger == null) {

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);
+}