Always include members of Null in subtype access.

Change-Id: I8e7fa2d418b9dd948b17b385b6d8e8e266582239
Reviewed-on: https://dart-review.googlesource.com/c/86566
Reviewed-by: Sigmund Cherem <[email protected]>
Commit-Queue: Johnni Winther <[email protected]>

Author: johnniwinther

pkg/compiler/lib/src/universe/resolution_world_builder.dart

@@ -989,6 +989,11 @@ class ResolutionWorldBuilderImpl extends WorldBuilderBase
         return _classHierarchyBuilder.isInheritedInThisClass(
             memberHoldingClass, type);
       case ClassRelation.subtype:
+        if (memberHoldingClass == _commonElements.nullClass ||
+            memberHoldingClass == _commonElements.jsNullClass) {
+          // Members of `Null` and `JSNull` are always potential targets.
+          return true;
+        }
         return _classHierarchyBuilder.isInheritedInSubtypeOf(
             memberHoldingClass, type);
     }

tests/compiler/dart2js_extra/881_test.dart

@@ -0,0 +1,13 @@
+// Copyright (c) 2018, 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.
+
+// Regression test for DartPad issue 881.
+
+void main() {
+  String v = null;
+  print('${v.hashCode}');
+  // Makes sure that [v] is not effectively final, so that we don't infer the
+  // static type from the initializer.
+  v = '';
+}