Use logger for polymer loader warning

pkg/polymer/lib/src/loader.dart

Index: pkg/polymer/lib/src/loader.dart
diff --git a/pkg/polymer/lib/src/loader.dart b/pkg/polymer/lib/src/loader.dart
index eb0a7269c46b61897f9ea52a6e8225981fce697e..47fa3108ddbce493472995e9056357d16d8ff0f3 100644
--- a/pkg/polymer/lib/src/loader.dart
+++ b/pkg/polymer/lib/src/loader.dart
@@ -141,6 +141,8 @@ final _rootUri = currentMirrorSystem().isolate.rootLibrary.uri;
 final String _packageRoot =
     '${path.dirname(Uri.parse(window.location.href).path)}/packages/';
 
+final Logger _loaderLog = new Logger('polymer.loader');
+
 /**
  * Reads the library at [uriString] (which can be an absolute URI or a relative
  * URI from the root library), and:
@@ -164,7 +166,7 @@ void _loadLibrary(String uriString) {
   }
 
   if (lib == null) {
-    print('warning: $uri library not found');
+    _loaderLog.info('$uri library not found');
     return;
   }