Add support for "??" to summaries.

pkg/analyzer/test/src/summary/summary_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 analyzer.test.src.summary.summary_common;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/error/listener.dart';
 import 'package:analyzer/src/dart/scanner/reader.dart';
@@ skipping 1738 matching lines @@
   test_constExpr_binary_or() {
     UnlinkedVariable variable =
         serializeVariableText('const v = false || true;');
     _assertUnlinkedConst(variable.initializer.bodyExpr, operators: [
       UnlinkedExprOperation.pushFalse,
       UnlinkedExprOperation.pushTrue,
       UnlinkedExprOperation.or
     ]);
   }
 
+  test_constExpr_binary_qq() {
+    UnlinkedVariable variable = serializeVariableText('const v = 1 ?? 2;');
+    _assertUnlinkedConst(variable.initializer.bodyExpr, operators: [
+      UnlinkedExprOperation.pushInt,
+      UnlinkedExprOperation.pushInt,
+      UnlinkedExprOperation.ifNull
+    ], ints: [
+      1,
+      2
+    ]);
+  }
+
   test_constExpr_binary_subtract() {
     UnlinkedVariable variable = serializeVariableText('const v = 1 - 2;');
     _assertUnlinkedConst(variable.initializer.bodyExpr, operators: [
       UnlinkedExprOperation.pushInt,
       UnlinkedExprOperation.pushInt,
       UnlinkedExprOperation.subtract
     ], ints: [
       1,
       2
     ]);
@@ skipping 8592 matching lines @@
 class _PrefixExpectation {
   final ReferenceKind kind;
   final String name;
   final String absoluteUri;
   final String relativeUri;
   final int numTypeParameters;
 
   _PrefixExpectation(this.kind, this.name,
       {this.absoluteUri, this.relativeUri, this.numTypeParameters: 0});
 }