Make dart2js accept constant null values in string interpolations.

pkg/compiler/lib/src/compile_time_constants.dart

Index: pkg/compiler/lib/src/compile_time_constants.dart
diff --git a/pkg/compiler/lib/src/compile_time_constants.dart b/pkg/compiler/lib/src/compile_time_constants.dart
index 1cc17a99d6c8ff7310c8e0f08bd0d9fcb625f78c..9558626a23f2870a03ad306e8b25f6cf00930755 100644
--- a/pkg/compiler/lib/src/compile_time_constants.dart
+++ b/pkg/compiler/lib/src/compile_time_constants.dart
@@ -508,7 +508,7 @@ class CompileTimeConstantEvaluator extends Visitor<AstConstant> {
       subexpressions.add(subexpression.expression);
       ConstantValue expression = subexpression.value;
       DartString expressionString;
-      if (expression.isNum || expression.isBool) {
+      if (expression.isNum || expression.isBool || expression.isNull) {
         PrimitiveConstantValue primitive = expression;
         expressionString =
             new DartString.literal(primitive.primitiveValue.toString());