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

tests/compiler/dart2js_native/native_null_closure_frog_test.dart

Index: tests/compiler/dart2js_native/native_null_closure_frog_test.dart
diff --git a/tests/compiler/dart2js_native/native_null_closure_frog_test.dart b/tests/compiler/dart2js_native/native_null_closure_frog_test.dart
index 5d1e96317ceee348c7d77df0c31dd7a3967196ec..1768cacc7fdd3a3e63400b3c08b0f5bc72034e5d 100644
--- a/tests/compiler/dart2js_native/native_null_closure_frog_test.dart
+++ b/tests/compiler/dart2js_native/native_null_closure_frog_test.dart
@@ -17,15 +17,18 @@ class A {
 
 makeA() native;
 
-void setup() native """
+void setup() {
+  JS('', r"""
+(function(){
 function A() {}
-A.prototype.setClosure = function(f) { this.f = f; };
-A.prototype.check = function(f) { return this.f === f; };
-A.prototype.invoke = function() { return this.f(); };
-makeA = function(){return new A;};
+  A.prototype.setClosure = function(f) { this.f = f; };
+  A.prototype.check = function(f) { return this.f === f; };
+  A.prototype.invoke = function() { return this.f(); };
+  makeA = function(){return new A()};
 
-self.nativeConstructor(A);
-""";
+  self.nativeConstructor(A);
+})()""");
+}
 
 main() {
   nativeTesting();