Fix an invalid variable name in DependencyQueue.

sdk/lib/_internal/pub/lib/src/solver/dependency_queue.dart

Index: sdk/lib/_internal/pub/lib/src/solver/dependency_queue.dart
diff --git a/sdk/lib/_internal/pub/lib/src/solver/dependency_queue.dart b/sdk/lib/_internal/pub/lib/src/solver/dependency_queue.dart
index 3f7c696903c07f7b35b45485de9938a10de8e9c8..e802154fe37e1571897fd821c73e79011ceb4178 100644
--- a/sdk/lib/_internal/pub/lib/src/solver/dependency_queue.dart
+++ b/sdk/lib/_internal/pub/lib/src/solver/dependency_queue.dart
@@ -131,7 +131,7 @@ class DependencyQueue {
       // If it fails for any reason, just treat that as no versions. This
       // will sort this reference higher so that we can traverse into it
       // and report the error more properly.
-      log.solver("Could not get versions for $dep:\n$error\n\n$trace");
+      log.solver("Could not get versions for $dep:\n$error\n\n$stackTrace");
       return 0;
     }