Fix an analyzer error in pub.

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

Index: sdk/lib/_internal/pub/lib/src/barback/web_socket_api.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/web_socket_api.dart b/sdk/lib/_internal/pub/lib/src/barback/web_socket_api.dart
index 7c9b30200a56beb9f0545539053877c0945bddb0..ff59dec41da26a232cc917dcd658fc153a8e0ca8 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/web_socket_api.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/web_socket_api.dart
@@ -272,10 +272,8 @@ class WebSocketApi {
 
     var urls = _environment.getUrlsForAssetPath(assetPath);
     if (urls.isEmpty) {
-      var name = '"$assetPath"';
-      if (command['package'] != null) name += ' in package "$package"';
       throw new _WebSocketException(_ErrorCode.NOT_SERVED,
-          'Asset path $name is not currently being served.');
+          'Asset path "$assetPath" is not currently being served.');
     }
 
     var result = {"urls": urls.map((url) => url.toString()).toList()};