Fix type inference of yield* statements

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

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// This file declares a "shadow hierarchy" of concrete classes which extend
 /// the kernel class hierarchy, adding methods and fields needed by the
 /// BodyBuilder.
 ///
 /// Instances of these classes may be created using the factory methods in
 /// `ast_factory.dart`.
@@ skipping 1306 matching lines @@
         closureContext.isGenerator ? closureContext.returnContext : null;
     if (isYieldStar && typeContext != null) {
       typeContext = inferrer.wrapType(
           typeContext,
           closureContext.isAsync
               ? inferrer.coreTypes.streamClass
               : inferrer.coreTypes.iterableClass);
     }
     var inferredType = inferrer.inferExpression(
         expression, typeContext, closureContext != null);
-    closureContext?.handleYield(inferrer, isYieldStar, inferredType);
+    closureContext.handleYield(inferrer, isYieldStar, inferredType);
     inferrer.listener.yieldStatementExit(this);
   }
 }