Pass in "mode" to transformer plugins.

pkg/polymer/lib/transformer.dart

Index: pkg/polymer/lib/transformer.dart
diff --git a/pkg/polymer/lib/transformer.dart b/pkg/polymer/lib/transformer.dart
index a4f0e10646a8549a9ed5126487f812e7506b6323..9bbd366256837fac7e68120c54f5befba117f91b 100644
--- a/pkg/polymer/lib/transformer.dart
+++ b/pkg/polymer/lib/transformer.dart
@@ -30,11 +30,12 @@ class PolymerTransformerGroup implements TransformerGroup {
   PolymerTransformerGroup(TransformOptions options)
       : phases = _createDeployPhases(options);
 
-  PolymerTransformerGroup.asPlugin(Map args) : this(_parseArgs(args));
+  PolymerTransformerGroup.asPlugin(BarbackSettings settings)
+      : this(_parseSettings(settings));
 }
 
-
-TransformOptions _parseArgs(Map args) {
+TransformOptions _parseSettings(BarbackSettings settings) {
+  var args = settings.transformer;
   return new TransformOptions(
       entryPoints: _readEntrypoints(args['entry_points']),
       directlyIncludeJS: args['js'] != false, // default to true