Fix type inference of yield* statements

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
index 05d7c6fc98b660d87af6576b8eadd98a50af3a95..de95a4dc9864c1e7f78665e11976f27ef9eaa73d 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
@@ -1324,7 +1324,7 @@ class KernelYieldStatement extends YieldStatement implements KernelStatement {
     }
     var inferredType = inferrer.inferExpression(
         expression, typeContext, closureContext != null);
-    closureContext?.handleYield(inferrer, isYieldStar, inferredType);
+    closureContext.handleYield(inferrer, isYieldStar, inferredType);
     inferrer.listener.yieldStatementExit(this);
   }
 }