Correct evaluation of interpolated `null`

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

Index: pkg/compiler/lib/src/constants/expressions.dart
diff --git a/pkg/compiler/lib/src/constants/expressions.dart b/pkg/compiler/lib/src/constants/expressions.dart
index a6317d5f9830ec8b4f83ca003228f7b309cf15c4..e3058661915fbe0583142d597365682120bfee33 100644
--- a/pkg/compiler/lib/src/constants/expressions.dart
+++ b/pkg/compiler/lib/src/constants/expressions.dart
@@ -663,7 +663,7 @@ class ConcatenateConstantExpression extends ConstantExpression {
     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());