fix #470, string interpolation now respects Dart toString

test/codegen/expect/methods.js

Index: test/codegen/expect/methods.js
diff --git a/test/codegen/expect/methods.js b/test/codegen/expect/methods.js
index 70f328fefea632ccac923edc084345d722443a07..0c12515309b317177407f5f809d25df561fff8ea 100644
--- a/test/codegen/expect/methods.js
+++ b/test/codegen/expect/methods.js
@@ -66,7 +66,7 @@ dart_library.library('methods', null, /* Imports */[
   });
   methods.Bar = class Bar extends core.Object {
     call(x) {
-      return core.print(`hello from ${x}`);
+      return core.print(dart.str`hello from ${x}`);
     }
   };
   dart.setSignature(methods.Bar, {