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

tests/compiler/dart2js_native/native_null_frog_test.dart

Index: tests/compiler/dart2js_native/native_null_frog_test.dart
diff --git a/tests/compiler/dart2js_native/native_null_frog_test.dart b/tests/compiler/dart2js_native/native_null_frog_test.dart
index 51a138b1c6f2ced53de08c8e327baa3ed00ba662..dad284afcd62d4cb2ce85362a4f4d70c01c961a3 100644
--- a/tests/compiler/dart2js_native/native_null_frog_test.dart
+++ b/tests/compiler/dart2js_native/native_null_frog_test.dart
@@ -16,15 +16,18 @@ class A {
 
 A makeA() native;
 
-void setup() native """
-function A() {}
-A.prototype.returnNull = function() { return null; };
-A.prototype.returnUndefined = function() { return void 0; };
-A.prototype.returnEmptyString = function() { return ""; };
-A.prototype.returnZero = function() { return 0; };
-makeA = function(){return new A;};
-self.nativeConstructor(A);
-""";
+void setup() {
+  JS('', r"""
+(function(){
+  function A() {}
+  A.prototype.returnNull = function() { return null; };
+  A.prototype.returnUndefined = function() { return void 0; };
+  A.prototype.returnEmptyString = function() { return ""; };
+  A.prototype.returnZero = function() { return 0; };
+  makeA = function(){return new A()};
+  self.nativeConstructor(A);
+})()""");
+}
 
 @NoInline()
 staticTests() {