Remove an unnecessary null check

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 2246 matching lines @@
 /// Concrete shadow object representing a yield statement in kernel form.
 class KernelYieldStatement extends YieldStatement implements KernelStatement {
   KernelYieldStatement(Expression expression, {bool isYieldStar: false})
       : super(expression, isYieldStar: isYieldStar);
 
   @override
   void _inferStatement(KernelTypeInferrer inferrer) {
     inferrer.listener.yieldStatementEnter(this);
     var closureContext = inferrer.closureContext;
     var typeContext =
         closureContext.isGenerator ? closureContext.returnContext : null;

[Paul Berry, 2017/06/15 16:33:34]

Note that the use of closureContext.isGenerator at this point in the function
guarantees that it is non-null below.

     if (isYieldStar && typeContext != null) {
       typeContext = inferrer.wrapType(
           typeContext,
           closureContext.isAsync
               ? 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);
   }
 }
 
 class _UnfinishedCascade extends Expression {
   accept(v) {
     return internalError("Internal error: Unsupported operation.");
   }
 
   accept1(v, arg) {
     return internalError("Internal error: Unsupported operation.");
   }
 
   getStaticType(types) {
     return internalError("Internal error: Unsupported operation.");
   }
 
   transformChildren(v) {
     return internalError("Internal error: Unsupported operation.");
   }
 
   visitChildren(v) {
     return internalError("Internal error: Unsupported operation.");
   }
 }