Allow user to specify mode for pub build/serve.

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

Index: sdk/lib/_internal/pub/lib/src/barback/dart_forwarding_transformer.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/dart_forwarding_transformer.dart b/sdk/lib/_internal/pub/lib/src/barback/dart_forwarding_transformer.dart
index 8c25a385b8f413fce1af16d3bd7f54b60b87f559..ed99c84c8f233a6818f6046ebb4b33efc0ec4757 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/dart_forwarding_transformer.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/dart_forwarding_transformer.dart
@@ -10,17 +10,27 @@ import 'package:barback/barback.dart';
 
 import '../utils.dart';
 
-/// A single transformer that just forwards any ".dart" file as an output.
+/// A single transformer that just forwards any ".dart" file as an output when
+/// not in release mode.
 ///
 /// Since the [Dart2JSTransformer] consumes its inputs, this is used in
 /// parallel to make sure the original Dart file is still available for use by
-/// Dartium.
+/// Dartium. In release mode, it's used to the do the exact opposite: it
+/// ensures that no Dart files are emitted, since all that is deployed is the
+/// compiled JavaScript.
 class DartForwardingTransformer extends Transformer {
+  /// The mode that the transformer is running in.
+  final BarbackMode _mode;
+
+  DartForwardingTransformer(this._mode);
+
   String get allowedExtensions => ".dart";
 
   Future apply(Transform transform) {
     return newFuture(() {
-      transform.addOutput(transform.primaryInput);
+      if (_mode != BarbackMode.RELEASE) {
+        transform.addOutput(transform.primaryInput);
+      }
     });
   }
 }