Add support for "??" to summaries.

pkg/analyzer/lib/src/summary/resynthesize.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.
 
 library summary_resynthesizer;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 478 matching lines @@
           break;
         case UnlinkedExprOperation.pushParameter:
           String name = uc.strings[stringPtr++];
           SimpleIdentifier identifier = AstFactory.identifier3(name);
           identifier.staticElement = _enclosingConstructor.parameters
               .firstWhere((parameter) => parameter.name == name,
                   orElse: () => throw new StateError(
                       'Unable to resolve constructor parameter: $name'));
           _push(identifier);
           break;
+        case UnlinkedExprOperation.ifNull:
+          _pushBinary(TokenType.QUESTION_QUESTION);
+          break;
         case UnlinkedExprOperation.assignToRef:
         case UnlinkedExprOperation.assignToProperty:
         case UnlinkedExprOperation.assignToIndex:
         case UnlinkedExprOperation.extractIndex:
         case UnlinkedExprOperation.invokeMethod:
         case UnlinkedExprOperation.cascadeSectionBegin:
         case UnlinkedExprOperation.cascadeSectionEnd:
         case UnlinkedExprOperation.typeCast:
         case UnlinkedExprOperation.typeCheck:
         case UnlinkedExprOperation.throwException:
@@ skipping 1368 matching lines @@
   static String _getElementIdentifier(String name, ReferenceKind kind) {
     if (kind == ReferenceKind.topLevelPropertyAccessor ||
         kind == ReferenceKind.propertyAccessor) {
       if (!name.endsWith('=')) {
         return name + '?';
       }
     }
     return name;
   }
 }