Use real "package:" imports in pub's plugin-loading code.

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

Index: sdk/lib/_internal/pub/lib/src/barback/server.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/server.dart b/sdk/lib/_internal/pub/lib/src/barback/server.dart
index 40d2c2cfce3bccfba5bc3d5b891b990be512c267..c918126f9ff32c2b5e96f7f8e60cf9422932cd6f 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/server.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/server.dart
@@ -150,7 +150,10 @@ class BarbackServer {
     }
 
     _logRequest(request, "Loading $id");
-    _environment.barback.getAssetById(id)
+    _environment.barback.getAssetById(id).then((result) {
+      _logRequest(request, "getAssetById($id) returned");
+      return result;
+    })
         .then((asset) => _serveAsset(request, asset))
         .catchError((error, trace) {
       if (error is! AssetNotFoundException) throw error;