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

pkg/kernel/lib/transformations/closure/rewriter.dart

Index: pkg/kernel/lib/transformations/closure/rewriter.dart
diff --git a/pkg/kernel/lib/transformations/closure/rewriter.dart b/pkg/kernel/lib/transformations/closure/rewriter.dart
index 6b962fd7b042b92da805e5857c22348db33e307e..6a73dfa5198e2928489a4435a55f49277179db4d 100644
--- a/pkg/kernel/lib/transformations/closure/rewriter.dart
+++ b/pkg/kernel/lib/transformations/closure/rewriter.dart
@@ -72,10 +72,12 @@ class BlockRewriter extends AstRewriter {
   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) {