dart2js_native tests: Migrate 'native STRING' to JS-calls

tests/compiler/dart2js_native/native_to_string_frog_test.dart

Index: tests/compiler/dart2js_native/native_to_string_frog_test.dart
diff --git a/tests/compiler/dart2js_native/native_to_string_frog_test.dart b/tests/compiler/dart2js_native/native_to_string_frog_test.dart
index 245553290ac44adaed229946eabffb19837b4be3..6fdca6f7c3fe7e4b5500cb5874bb0d48992f9d23 100644
--- a/tests/compiler/dart2js_native/native_to_string_frog_test.dart
+++ b/tests/compiler/dart2js_native/native_to_string_frog_test.dart
@@ -11,11 +11,14 @@ class A {
 
 makeA() native;
 
-void setup() native """
-function A() {}
-makeA = function(){return new A;};
-self.nativeConstructor(A);
-""";
+void setup() {
+  JS('', r"""
+(function(){
+  function A() {}
+  makeA = function(){return new A()};
+  self.nativeConstructor(A);
+})()""");
+}
 
 main() {
   nativeTesting();