Fix an invalid variable name in DependencyQueue.

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

Index: sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
diff --git a/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart b/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
index 743ef038dfbed8c5680caff8f609ed2d3475da7b..66a7b9066bca8859ba3e397b684dcd148e0966a5 100644
--- a/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
+++ b/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
@@ -504,7 +504,7 @@ class Traverser {
     var pubspec;
     try {
       pubspec = await _solver.cache.getPubspec(id);
-    } on PackageNotFoundException catch (error) {
+    } on PackageNotFoundException {
       // We can only get here if the lockfile refers to a specific package
       // version that doesn't exist (probably because it was yanked).
       throw new NoVersionException(id.name, null, id.version, []);