Add support for "??" to summaries.

pkg/analyzer/lib/src/summary/summarize_const_expr.dart

 // Copyright (c) 2016, 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 serialization.summarize_const_expr;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/element/type.dart' show DartType;
 import 'package:analyzer/src/summary/format.dart';
@@ skipping 468 matching lines @@
     } else if (operator == TokenType.GT) {
       operations.add(UnlinkedExprOperation.greater);
     } else if (operator == TokenType.LT) {
       operations.add(UnlinkedExprOperation.less);
     } else if (operator == TokenType.GT_EQ) {
       operations.add(UnlinkedExprOperation.greaterEqual);
     } else if (operator == TokenType.LT_EQ) {
       operations.add(UnlinkedExprOperation.lessEqual);
     } else if (operator == TokenType.PERCENT) {
       operations.add(UnlinkedExprOperation.modulo);
+    } else if (operator == TokenType.QUESTION_QUESTION) {
+      operations.add(UnlinkedExprOperation.ifNull);
     } else {
       throw new StateError('Unknown operator: $operator');
     }
   }
 
   void _serializeCascadeExpression(CascadeExpression expr) {
     _serialize(expr.target);
     for (Expression section in expr.cascadeSections) {
       operations.add(UnlinkedExprOperation.cascadeSectionBegin);
       _serialize(section);
@@ skipping 143 matching lines @@
     if (typeArguments == null) {
       ints.add(0);
     } else {
       ints.add(typeArguments.arguments.length);
       for (TypeName typeName in typeArguments.arguments) {
         references.add(serializeTypeName(typeName));
       }
     }
   }
 }