Remove an unnecessary null check

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 5eea63328e1fb554cb1da106f990624b210683e0..f62dac93da4edee221530816de912d2ac7f68500 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
@@ -2272,8 +2272,7 @@ class KernelYieldStatement extends YieldStatement implements KernelStatement {
               ? inferrer.coreTypes.streamClass
               : inferrer.coreTypes.iterableClass);
     }
-    var inferredType = inferrer.inferExpression(
-        expression, typeContext, closureContext != null);
+    var inferredType = inferrer.inferExpression(expression, typeContext, true);
     closureContext.handleYield(inferrer, isYieldStar, inferredType);
     inferrer.listener.yieldStatementExit(this);
   }