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

tests/compiler/dart2js_native/native_exception2_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.
 
 // Similar to native_exception_test.dart but also defines a native
 // class.  This exercises a different code path in emitter.dart.
 
 library native_exception2_test;
 
 import 'native_exception_test.dart' as other;
 import 'native_testing.dart';
 
 @Native("NativeClass")
 class NativeClass {
   foo() => 'oof';
 }
 
 makeNativeClass() native;
 
-setup() native """
-function NativeClass() {}
-makeNativeClass = function() { return new NativeClass; }
-self.nativeConstructor(NativeClass);
-""";
+setup() {
+  JS('', r"""
+(function(){
+  function NativeClass() {}
+  makeNativeClass = function() { return new NativeClass(); };
+  self.nativeConstructor(NativeClass);
+})()""");
+}
 
 main() {
   nativeTesting();
   setup();
   Expect.equals('oof', makeNativeClass().foo());
   other.main();
 }