fix #470, string interpolation now respects Dart toString

lib/src/compiler/code_generator.dart

Index: lib/src/compiler/code_generator.dart
diff --git a/lib/src/compiler/code_generator.dart b/lib/src/compiler/code_generator.dart
index 05d603f8de8c428c91f543740d95d541006ad1c7..69805bf75e416ed6be14f1faef765172408e6c2e 100644
--- a/lib/src/compiler/code_generator.dart
+++ b/lib/src/compiler/code_generator.dart
@@ -4188,11 +4188,9 @@ class CodeGenerator extends GeneralizingAstVisitor
       _visitListToBinary(node.strings, '+');
 
   @override
-  JS.TemplateString visitStringInterpolation(StringInterpolation node) {
-    // Assuming we implement toString() on our objects, we can avoid calling it
-    // in most cases. Builtin types may differ though. We could handle this with
-    // a tagged template.
-    return new JS.TemplateString(_visitList(node.elements));
+  JS.Expression visitStringInterpolation(StringInterpolation node) {
+    return new JS.TaggedTemplate(
+        js.call('dart.str'), new JS.TemplateString(_visitList(node.elements)));
   }
 
   @override