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

tests/compiler/dart2js_native/native_field_invocation3_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";
 
 makeCC() native;
 
-void setup() native """
-function CC() {}
-makeCC = function() { return new CC; }
-self.nativeConstructor(CC);
-""";
+void setup() {
+  JS('', r"""
+(function(){
+  function CC() {}
+  makeCC = function() { return new CC() };
+  self.nativeConstructor(CC);
+})()""");
+}
 
 @Native("CC")
 class ClickCounter {
   var status;
 
   var foo;
 
   init() {
     foo = wrap(g);
   }
 
   g(val) => "### $val ###";
 }
 
 wrap(cb) {
   return (val) {
     return cb("!!! $val !!!");
   };
 }
 
 main() {
   nativeTesting();
   setup();
   var c = makeCC();
   c.init();
   // `foo` contains a closure. Make sure that invoking foo through an
   // interceptor works.
   Expect.equals("### !!! 499 !!! ###", c.foo(499));
 }