Fix incorrect nullability inference for ??

pkg/dev_compiler/lib/src/compiler/nullable_type_inference.dart

 // Copyright (c) 2015, 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.
 
 import 'dart:collection';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
 import 'package:analyzer/dart/ast/token.dart' show TokenType;
 import 'package:analyzer/dart/ast/visitor.dart' show RecursiveAstVisitor;
 import 'package:analyzer/dart/element/element.dart';
@@ skipping 41 matching lines @@
   ///
   /// If [localIsNullable] is not supplied, this will use the known list of
   /// [_notNullLocals].
   bool _isNullable(Expression expr,
       [bool localIsNullable(LocalVariableElement e)]) {
     // TODO(jmesserly): we do recursive calls in a few places. This could
     // leads to O(depth) cost for calling this function. We could store the
     // resulting value if that becomes an issue, so we maintain the invariant
     // that each node is visited once.
     Element element = null;
-    if (expr is PropertyAccess) {
+    if (expr is PropertyAccess &&
+        expr.operator?.type != TokenType.QUESTION_PERIOD) {
       element = expr.propertyName.staticElement;
     } else if (expr is Identifier) {
       element = expr.staticElement;
     }
     if (element != null) {
       // Type literals are not null.
       if (element is ClassElement || element is FunctionTypeAliasElement) {
         return false;
       }
 
@@ skipping 254 matching lines @@
           return false;
         }
 
         if (_nullInference._isNullable(right, visitLocal)) {
           _nullableLocals.add(element);
         }
       }
     }
   }
 }