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

tests/compiler/dart2js_native/native_mixin_multiple2_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";
 
 // Test that native classes can access methods defined only by mixins.
 
 @Native("A")
 class A {
   foo(x, [y]) => '$x;$y';
 }
 
 @Native("B")
 class B extends A with M1, M2, M3 {}
 
 class M1 {}
 
 class M2 {
   // These methods are only defined in this non-first, non-last mixin.
   plain(x) => 'P $x';
   bar(x, [y]) => '$y,$x';
 }
 
 class M3 {}
 
 makeB() native;
 
-void setup() native """
-function B() {}
-makeB = function(){return new B;};
+void setup() {
+  JS('', r"""
+(function(){
+  function B() {}
+  makeB = function(){return new B()};
 
-self.nativeConstructor(B);
-""";
+  self.nativeConstructor(B);
+})()""");
+}
 
 main() {
   nativeTesting();
   setup();
 
   B b = makeB();
   Expect.equals('1;2', b.foo(1, 2));
   Expect.equals('2;null', b.foo(2));
   Expect.equals('P 3', b.plain(3));
   Expect.equals('100,4', b.bar(4, 100));
   Expect.equals('null,5', b.bar(5));
 }