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

tests/compiler/dart2js_native/native_field_invocation6_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";
 
 makeA() native;
 nativeFirst(x, y) native;
 
-void setup() native """
-nativeFirst = function(x, y) { return x; }
+void setup() {
+  JS('', r"""
+(function(){
+  nativeFirst = function(x, y) { return x; };
 
-function A() {}
-makeA = function() { return new A; }
-self.nativeConstructor(A);
-""";
+  function A() {}
+  makeA = function() { return new A() };
+  self.nativeConstructor(A);
+})()""");
+}
 
 @Native("A")
 class A {
   var _foo;
 
   get foo => _foo;
 
   init() {
     _foo = () => 42;
   }
@@ skipping 19 matching lines @@
   var b = new B();
   var c = new C();
   var d = new D();
   a = nativeFirst(a, b);
   a = nativeFirst(a, c);
   a = nativeFirst(a, d);
   // The variable a is still an instance of class A, but dart2js can't infer
   // that anymore.
   Expect.equals(42, a.foo());
 }