Better progress reporting.

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

Index: sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart b/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
index de1e66c331a779b4b8180fc5a0e44e6ed960174e..99b3919556d7d4a30415c24eaa59a2063c2f5447 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
@@ -233,7 +233,7 @@ class BuildEnvironment {
     return BarbackServer.bind(this, _hostname, 0, null).then((server) {
       transformerServer = server;
 
-      return _provideSources();
+      return log.progress("Loading source assets", _provideSources);
     }).then((_) {
       log.fine("Provided sources.");
       var completer = new Completer();