dart2js_native tests: Migrate 'native STRING' to JS-calls - part 2/2

tests/compiler/dart2js_native/subclassing_3_test.dart

 // Copyright (c) 2013, 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';
 import 'dart:_js_helper' show setNativeSubclassDispatchRecord;
 import 'dart:_interceptors' show Interceptor, findInterceptorForType;
 
 // Test calling convention of methods introduced on subclasses of native
 // class of mixin.
@@ skipping 16 matching lines @@
   // have interceptor calling convention.
   oof() => 'B';
   // [miz] is introduced only on the mixin-application A+M.
 }
 
 B makeB() native;
 
 @Creates('=Object')
 getBPrototype() native;
 
-void setup() native r"""
-function B() {}
-makeB = function(){return new B;};
+void setup() {
+  JS('', r"""
+(function(){
+  function B() {}
+  makeB = function(){return new B();};
 
-getBPrototype = function(){return B.prototype;};
-""";
+  getBPrototype = function(){return B.prototype;};
+})()""");
+}
 
 main() {
   nativeTesting();
   setup();
 
   setNativeSubclassDispatchRecord(getBPrototype(), findInterceptorForType(B));
 
   B b = makeB();
   Expect.equals('1,B,M', b.foo(1));
 }