Correct evaluation of interpolated `null`

tests/compiler/dart2js/constant_expression_evaluate_test.dart

Index: tests/compiler/dart2js/constant_expression_evaluate_test.dart
diff --git a/tests/compiler/dart2js/constant_expression_evaluate_test.dart b/tests/compiler/dart2js/constant_expression_evaluate_test.dart
index 9b5adcf2cc07e7d07e565910a0cf1e77cf497b07..3dd05aca7e278388a528581b03437ac31ad5d2ef 100644
--- a/tests/compiler/dart2js/constant_expression_evaluate_test.dart
+++ b/tests/compiler/dart2js/constant_expression_evaluate_test.dart
@@ -61,6 +61,8 @@ const List<TestData> DATA = const [
     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)' }),