Log when dart2js starts compiling.

sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart

Index: sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
index 84ca33618cb4c4b08c8ed2085ecb4d744ab9eeb9..7c7f53944b6357219fc6f4412ac444b1acfa60f9 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
@@ -70,6 +70,8 @@ class Dart2JSTransformer extends Transformer implements LazyTransformer {
   }
 
   Future apply(Transform transform) {
+    transform.logger.info("Compiling ${transform.primaryInput.id}...");
+
     // Wait for any ongoing apply to finish first.
     // TODO(rnystrom): If there are multiple simultaneous compiles, this will
     // resume and pause them repeatedly. It still serializes them correctly,