fix typo in future union inference code

pkg/analyzer/lib/src/generated/type_system.dart

Index: pkg/analyzer/lib/src/generated/type_system.dart
diff --git a/pkg/analyzer/lib/src/generated/type_system.dart b/pkg/analyzer/lib/src/generated/type_system.dart
index 2462145233f78bcb4af528a3558501a6c9dc7ba2..c8c1fe652f592e4f2370799c199f87c3a6652164 100644
--- a/pkg/analyzer/lib/src/generated/type_system.dart
+++ b/pkg/analyzer/lib/src/generated/type_system.dart
@@ -1595,7 +1595,7 @@ class _StrongInferenceTypeSystem extends StrongTypeSystemImpl {
         //
         // This will typically lead to top with the current rules, but it will
         // work with `bottom` or if we remove Future flattening.
-        var f = upperBound as FutureUnionType;
+        var f = lowerBound as FutureUnionType;
         lowerBound = _typeSystem.getLeastUpperBound(
             _typeProvider, f.futureOfType, f.type);
       }