Correct evaluation of interpolated `null`

pkg/compiler/lib/src/constants/expressions.dart

 // Copyright (c) 2014, 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.constants.expressions;
 
 import '../common.dart';
 import '../constants/constant_system.dart';
 import '../core_types.dart';
 import '../dart_types.dart';
@@ skipping 645 matching lines @@
         expressions.map((a) => a.apply(arguments)).toList());
   }
 
   @override
   ConstantValue evaluate(
       Environment environment, ConstantSystem constantSystem) {
     DartString accumulator;
     for (ConstantExpression expression in expressions) {
       ConstantValue value = expression.evaluate(environment, constantSystem);
       DartString valueString;
-      if (value.isNum || value.isBool) {
+      if (value.isNum || value.isBool || value.isNull) {
         PrimitiveConstantValue primitive = value;
         valueString =
             new DartString.literal(primitive.primitiveValue.toString());
       } else if (value.isString) {
         PrimitiveConstantValue primitive = value;
         valueString = primitive.primitiveValue;
       } else {
         // TODO(johnniwinther): Specialize message to indicated that the problem
         // is not constness but the types of the const expressions.
         return new NonConstantValue();
@@ skipping 1214 matching lines @@
     visit(exp.name);
     if (exp.defaultValue != null) {
       sb.write(', defaultValue: ');
       visit(exp.defaultValue);
     }
     sb.write(')');
   }
 
   String toString() => sb.toString();
 }