Handle constant ?? expressions in constructors.

tests/compiler/dart2js/constant_expression_test.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 constant_expression_test;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/constants/expressions.dart';
@@ skipping 35 matching lines @@
   const TestData('', const [
     const ConstantData('null', ConstantExpressionKind.NULL),
     const ConstantData('false', ConstantExpressionKind.BOOL),
     const ConstantData('true', ConstantExpressionKind.BOOL),
     const ConstantData('0', ConstantExpressionKind.INT),
     const ConstantData('0.0', ConstantExpressionKind.DOUBLE),
     const ConstantData('"foo"', ConstantExpressionKind.STRING),
     const ConstantData('1 + 2', ConstantExpressionKind.BINARY),
     const ConstantData('1 == 2', ConstantExpressionKind.BINARY),
     const ConstantData('1 != 2', ConstantExpressionKind.BINARY),
+    const ConstantData('1 ?? 2', ConstantExpressionKind.BINARY),
     const ConstantData('-(1)', ConstantExpressionKind.UNARY, text: '-1'),
     const ConstantData('"foo".length', ConstantExpressionKind.STRING_LENGTH),
     const ConstantData('identical(0, 1)', ConstantExpressionKind.IDENTICAL),
     const ConstantData('"a" "b"', ConstantExpressionKind.CONCATENATE,
                        text: '"ab"'),
     const ConstantData('identical', ConstantExpressionKind.FUNCTION),
     const ConstantData('true ? 0 : 1', ConstantExpressionKind.CONDITIONAL),
     const ConstantData('proxy', ConstantExpressionKind.VARIABLE),
     const ConstantData('Object', ConstantExpressionKind.TYPE),
     const ConstantData('#name', ConstantExpressionKind.SYMBOL),
@@ skipping 173 matching lines @@
         String name = '$field';
         String expression = instanceFields[field].toDartText();
         String expected = data.fields[name];
         Expect.equals(expected, expression,
             "Unexpected field expression ${expression} for field '$name' in "
             "contant `${constant.toDartText()}`, expected '${expected}'.");
       });
     }
   });
 }