Add support for "??" to summaries.

pkg/analyzer/test/src/summary/resynthesize_common.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 test.src.serialization.elements_test;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/constant/value.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
@@ skipping 2285 matching lines @@
 const vConditional = (1 == 2) ? 11 : 22;
 ''');
   }
 
   test_const_topLevel_identical() {
     checkLibrary(r'''
 const vIdentical = (1 == 2) ? 11 : 22;
 ''');
   }
 
+  test_const_topLevel_ifNull() {
+    checkLibrary(r'''
+const vIfNull = 1 ? 2.0;

[scheglov, 2016/11/22 17:51:37]

`1 ?? 2.0`

[Paul Berry, 2016/11/22 18:02:58]

Thank you!  Fixed.

I'm surprised this didn't lead to a test failure--it looks like resynthesis
tests no longer check for errors.  I will submit a follow-up CL which fixes
that.

+''');
+  }
+
   test_const_topLevel_literal() {
     checkLibrary(r'''
 const vNull = null;
 const vBoolFalse = false;
 const vBoolTrue = true;
 const vInt = 1;
 const vIntLong = 0x9876543210987654321;
 const vDouble = 2.3;
 const vString = 'abc';
 const vStringConcat = 'aaa' 'bbb';
@@ skipping 2426 matching lines @@
       fail('Unexpectedly tried to get unlinked summary for $uri');
     }
     return serializedUnit;
   }
 
   @override
   bool hasLibrarySummary(String uri) {
     return true;
   }
 }