Ensure that errors have stack traces attached.

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 4c8cf596217a0739367a8d423f812fd248f22ffc..7fd758f99433ce53f6094817d5e63fe47a9c06ca 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/server.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/server.dart
@@ -130,7 +130,7 @@ class BarbackServer {
         trace = new Trace.from(trace);
         _logRequest(request, "$error\n$trace");
 
-        _resultsController.addError(error);
+        _resultsController.addError(error, trace);
         close();
         return;
       }
@@ -208,12 +208,8 @@ class BarbackServer {
             _webSocketError(socket, 'Unknown command "${command["command"]}".');
             break;
         }
-      }, onError: (error) {
-        _resultsController.addError(error);
-      }, cancelOnError: true);
-    }).catchError((error) {
-      _resultsController.addError(error);
-    });
+      }, onError: _resultsController.addError, cancelOnError: true);
+    }).catchError(_resultsController.addError);
   }
 
   /// Converts a [url] served by pub serve into an [AssetId] that can be