Pass in "mode" to transformer plugins.

sdk/lib/_internal/pub/lib/src/command/serve.dart

Index: sdk/lib/_internal/pub/lib/src/command/serve.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command/serve.dart b/sdk/lib/_internal/pub/lib/src/command/serve.dart
index 0d7adfc361ea8e652b9718fb0258b73913665067..d3df35a2563f8e81aa1c8b9814d54db966261022 100644
--- a/sdk/lib/_internal/pub/lib/src/command/serve.dart
+++ b/sdk/lib/_internal/pub/lib/src/command/serve.dart
@@ -6,6 +6,8 @@ library pub.command.serve;
 
 import 'dart:async';
 
+import 'package:barback/barback.dart';
+
 import '../barback/dart_forwarding_transformer.dart';
 import '../barback/dart2js_transformer.dart';
 import '../barback/pub_package_provider.dart';
@@ -75,7 +77,8 @@ class ServeCommand extends PubCommand {
         ];
       }
 
-      return barback.createServer(hostname, port, graph,
+      // TODO(rnystrom): Allow specifying other modes.
+      return barback.createServer(hostname, port, graph, BarbackMode.DEBUG,
           builtInTransformers: builtInTransformers);
     }).then((server) {
       /// This completer is used to keep pub running (by not completing) and