Correct evaluation of interpolated `null`

tests/compiler/dart2js/constant_expression_evaluate_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 dart2js.constants.expressions.evaluate_test;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/constants/evaluation.dart';
@@ skipping 43 matching lines @@
     const ConstantData('0.0', const { const {} : 'DoubleConstant(0.0)' }),
     const ConstantData('"foo"', const { const {} : 'StringConstant("foo")' }),
     const ConstantData('1 + 2', const { const {} : 'IntConstant(3)' }),
     const ConstantData('-(1)', const { const {} : 'IntConstant(-1)' }),
     const ConstantData('1 == 2', const { const {} : 'BoolConstant(false)' }),
     const ConstantData('1 != 2', const { const {} : 'BoolConstant(true)' }),
     const ConstantData('"foo".length', const { const {} : 'IntConstant(3)' }),
     const ConstantData('identical(0, 1)',
                        const { const {} : 'BoolConstant(false)' }),
     const ConstantData('"a" "b"', const { const {} : 'StringConstant("ab")' }),
+    const ConstantData(r'"${null}"',
+        const { const {} : 'StringConstant("null")' }),
     const ConstantData('identical',
         const { const {} : 'FunctionConstant(identical)' }),
     const ConstantData('true ? 0 : 1', const { const {} : 'IntConstant(0)' }),
     const ConstantData('proxy',
         const { const {} : 'ConstructedConstant(_Proxy())' }),
     const ConstantData('Object', const { const {} : 'TypeConstant(Object)' }),
     const ConstantData('const [0, 1]',
         const { const {} : 'ListConstant([IntConstant(0), IntConstant(1)])' }),
     const ConstantData('const <int>[0, 1]', const {
         const {} : 'ListConstant(<int>[IntConstant(0), IntConstant(1)])' }),
@@ skipping 166 matching lines @@
       Environment environment = new MemoryEnvironment(compiler, env);
       ConstantValue value =
           constant.evaluate(environment, DART_CONSTANT_SYSTEM);
       String valueText = value.toStructuredText();
       Expect.equals(expectedText, valueText,
           "Unexpected value '${valueText}' for contant "
           "`${constant.toDartText()}`, expected '${expectedText}'.");
     });
   });
 }