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

tests/compiler/dart2js_native/native_checked_fields_frog_test.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'native_testing.dart';
 
 // Test that type checks occur on assignment to fields of native methods.
 
 @Native("A")
 class A {
   int foo;
 }
 
 @Native("B")
 class B {
   String foo;
 }
 
 A makeA() native;
 B makeB() native;
 
-void setup() native """
-function A() {}
+void setup() {
+  JS('', r"""
+(function(){
+  function A() {}
 
-function B() {}
+  function B() {}
 
-makeA = function(){return new A;};
-makeB = function(){return new B;};
+  makeA = function(){return new A()};
+  makeB = function(){return new B()};
 
-self.nativeConstructor(A);
-self.nativeConstructor(B);
-""";
+  self.nativeConstructor(A);
+  self.nativeConstructor(B);
+})()""");
+}
 
 expectThrows(action()) {
   bool threw = false;
   try {
     action();
   } catch (e) {
     threw = true;
   }
   Expect.isTrue(threw);
 }
@@ skipping 70 matching lines @@
 main() {
   nativeTesting();
   setup();
 
   if (isCheckedMode()) {
     checkedModeTest();
   } else {
     uncheckedModeTest();
   }
 }