closure conversion: Do not set Context.parent if the value is `null`

pkg/kernel/lib/transformations/closure/rewriter.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.
 
 library kernel.transformations.closure.rewriter;
 
 import '../../ast.dart';
 import 'converter.dart' show ClosureConverter;
 
 /// Used by the [Context] to initialize and update the context variable
@@ skipping 54 matching lines @@
   }
 
   void _insertStatement(Statement statement) {
     _currentBlock.statements.insert(_insertionIndex++, statement);
     statement.parent = _currentBlock;
   }
 
   void insertContextDeclaration(Class contextClass, Expression accessParent) {
     _createDeclaration(contextClass);
     _insertStatement(contextDeclaration);
-    _insertStatement(new ExpressionStatement(new PropertySet(
-        new VariableGet(contextDeclaration),
-        new Name('parent'),
-        accessParent)));
+    if (accessParent is! NullLiteral) {
+      _insertStatement(new ExpressionStatement(new PropertySet(
+          new VariableGet(contextDeclaration),
+          new Name('parent'),
+          accessParent)));
+    }
   }
 
   void insertExtendContext(Expression accessContext, Arguments arguments) {
     _insertStatement(new ExpressionStatement(
         new MethodInvocation(accessContext, new Name('[]='), arguments)));
   }
 
   Block _currentBlock;
   int _insertionIndex;
 }
@@ skipping 27 matching lines @@
     Expression extendContext =
         new MethodInvocation(accessContext, new Name('[]='), arguments);
     Let parent = initializingExpression.parent;
     Let binding = new Let(
         new VariableDeclaration(null, initializer: extendContext),
         initializingExpression);
     parent.body = binding;
     binding.parent = parent;
   }
 }