Add instrumentation for tracking how long it takes barback to start.

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

Index: sdk/lib/_internal/pub/lib/src/barback/barback_server.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/barback_server.dart b/sdk/lib/_internal/pub/lib/src/barback/barback_server.dart
index d53d3eafbcd5dfbf87180b812191860dcd4e119c..340b4a28f05842acb930afba6daff8b7b61548f9 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/barback_server.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/barback_server.dart
@@ -101,9 +101,7 @@ class BarbackServer extends BaseServer<BarbackServerResult> {
           asset: id);
     }
 
-    logRequest(request, "Loading $id");
     return environment.barback.getAssetById(id).then((result) {
-      logRequest(request, "getAssetById($id) returned");
       return result;
     }).then((asset) => _serveAsset(request, asset)).catchError((error, trace) {
       if (error is! AssetNotFoundException) throw error;