Pass in "mode" to transformer plugins.

sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart

Index: sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
index 621e85a1bb7df2446bc9f1be9c67079eeabdc9b1..714fd70b645e836d95d09e0591473df0c196844d 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
@@ -39,7 +39,10 @@ void main(_, SendPort replyTo) {
     _respond(wrappedMessage, (message) {
       var library = Uri.parse(message['library']);
       var configuration = JSON.decode(message['configuration']);
-      return initialize(library, configuration).
+      if (configuration == null) configuration = {};
+      var mode = new BarbackMode.fromString(message['mode']);
+      var settings = new BarbackSettings(configuration, mode);
+      return initialize(library, settings).
           map(_serializeTransformerOrGroup).toList();
     });
   });
@@ -48,9 +51,8 @@ void main(_, SendPort replyTo) {
 /// Loads all the transformers and groups defined in [uri].
 ///
 /// Loads the library, finds any Transformer or TransformerGroup subclasses in
-/// it, instantiates them (with [configuration] if it's non-null), and returns
-/// them.
-Iterable initialize(Uri uri, Map configuration) {
+/// it, instantiates them with [settings], and returns them.
+Iterable initialize(Uri uri, BarbackSettings settings) {
   var mirrors = currentMirrorSystem();
   var transformerClass = reflectClass(Transformer);
   var groupClass = reflectClass(TransformerGroup);
@@ -67,19 +69,12 @@ Iterable initialize(Uri uri, Map configuration) {
 
     var constructor = getConstructor(classMirror, 'asPlugin');
     if (constructor == null) return null;
-    if (constructor.parameters.isEmpty) {
-      if (configuration != null) return null;
-      return classMirror.newInstance(const Symbol('asPlugin'), []).reflectee;
-    }
-    if (constructor.parameters.length != 1) return null;
+    if (constructor.parameters.length > 1) return null;
 
-    // If the constructor expects configuration and none was passed, it defaults
-    // to an empty map.
-    if (configuration == null) configuration = {};
+    var parameters = [];
+    if (constructor.parameters.isNotEmpty) parameters.add(settings);
 
-    // TODO(nweiz): if the constructor accepts named parameters, automatically
-    // destructure the configuration map.
-    return classMirror.newInstance(const Symbol('asPlugin'), [configuration])
+    return classMirror.newInstance(const Symbol('asPlugin'), parameters)
         .reflectee;
   }).where((classMirror) => classMirror != null);
 }
@@ -444,7 +439,8 @@ Stream callbackStream(Stream callback()) {
 /// [id].
 ///
 /// [server] is used to serve any Dart files needed to load the transformers.
-Future<Set> loadTransformers(BarbackServer server, TransformerId id) {
+Future<Set> loadTransformers(BarbackServer server, BarbackMode mode,
+    TransformerId id) {
   return id.getAssetId(server.barback).then((assetId) {
     var path = assetId.path.replaceFirst('lib/', '');
     // TODO(nweiz): load from a "package:" URI when issue 12474 is fixed.
@@ -460,6 +456,7 @@ Future<Set> loadTransformers(BarbackServer server, TransformerId id) {
         .then((sendPort) {
       return _call(sendPort, {
         'library': uri,
+        'mode': mode.name,
         // TODO(nweiz): support non-JSON-encodable configuration maps.
         'configuration': JSON.encode(id.configuration)
       }).then((transformers) {
@@ -552,7 +549,7 @@ Map _serializeTransform(Transform transform) {
 
       if (message['type'] == 'addOutput') {
         transform.addOutput(_deserializeAsset(message['output']));
-        return;
+        return null;
       }
 
       assert(message['type'] == 'log');