Handle ?? in Constantifier

tests/compiler/dart2js/kernel/impact_test.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 dart2js.kernel.impact_test;
 
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/common.dart';
 import 'package:compiler/src/common/names.dart';
@@ skipping 39 matching lines @@
   testFalse();
   testInt();
   testDouble();
   testString();
   testStringInterpolation();
   testStringInterpolationConst();
   testStringJuxtaposition();
   testSymbol();
   testConstSymbol();
   testComplexConstSymbol();
+  testIfNullConstSymbol();
   testTypeLiteral();
   testBoolFromEnvironment();
   testEmptyListLiteral();
   testEmptyListLiteralDynamic();
   testEmptyListLiteralTyped();
   testEmptyListLiteralConstant();
   testNonEmptyListLiteral();
   testEmptyMapLiteral();
   testEmptyMapLiteralDynamic();
   testEmptyMapLiteralTyped();
@@ skipping 157 matching lines @@
   const <int>[]: const String.fromEnvironment('c'),
   testComplexConstSymbol: #testComplexConstSymbol,
   1 + 2: identical(0, -0), 
   true || false: false && true,
   override: const GenericClass<int, String>.generative(),
 }; 
 const complexSymbolField = 
     complexSymbolField1 ? complexSymbolField2 : complexSymbolField3;
 testComplexConstSymbol() => const Symbol(complexSymbolField);
 
+testIfNullConstSymbol() => const Symbol(null ?? 'foo');
+
 testTypeLiteral() => Object;
 testBoolFromEnvironment() => const bool.fromEnvironment('FOO');
 testEmptyListLiteral() => [];
 testEmptyListLiteralDynamic() => <dynamic>[];
 testEmptyListLiteralTyped() => <String>[];
 testEmptyListLiteralConstant() => const [];
 testNonEmptyListLiteral() => [0];
 testEmptyMapLiteral() => {};
 testEmptyMapLiteralDynamic() => <dynamic, dynamic>{};
 testEmptyMapLiteralTyped() => <String, int>{};
@@ skipping 689 matching lines @@
         visitList(type.optionalParameterTypes),
         type.namedParameters,
         visitList(type.namedParameterTypes));
   }
 }
 
 /// Perform unaliasing of all typedefs nested within a [ResolutionDartType].
 ResolutionDartType unalias(ResolutionDartType type) {
   return const Unaliaser().visit(type);
 }