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

tests/compiler/dart2js_native/native_field_invocation2_test.dart

 // Copyright (c) 2014, 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";
 
 @Native("NNative")
 class NNative {
   var status;
 
@@ skipping 15 matching lines @@
 }
 
 wrap(cb) {
   return (val) {
     return cb("!!! $val !!!");
   };
 }
 
 nativeId(x) native;
 
-void setup() native """
+void setup() {
+  JS('', r"""
+(function(){
 nativeId = function(x) { return x; }
-""";
+})()""");
+}
 
 main() {
   setup();
   // Make sure the ClickCounter class goes through interceptors.
   Expect.equals("### !!! 42 !!! ###", nativeId(new ClickCounter()).f(42));
   // We are interested in the direct call, where the type is known.
   Expect.equals("### !!! 499 !!! ###", new ClickCounter().f(499));
 }