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

tests/compiler/dart2js_native/native_mixin_with_plain_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 and ordinary Dart classes can both use the same
 // ordinary Dart classes as a mixin.
 
 @Native("A")
@@ skipping 22 matching lines @@
 }
 
 class D extends C with M {
   bar() => 'D-bar -> ${baz()}';
   get mm => 'D.mm($cc)';
 }
 
 makeA() native;
 makeB() native;
 
-void setup() native """
-function A() {this.aa = 'aa'}
-function B() {this.aa = 'bb'}
-makeA = function(){return new A;};
-makeB = function(){return new B;};
+void setup() {
+  JS('', r"""
+(function(){
+  function A() {this.aa = 'aa'}
+  function B() {this.aa = 'bb'}
+  makeA = function(){return new A()};
+  makeB = function(){return new B()};
 
-self.nativeConstructor(A);
-self.nativeConstructor(B);
-""";
+  self.nativeConstructor(A);
+  self.nativeConstructor(B);
+})()""");
+}
 
 main() {
   nativeTesting();
   setup();
   var things = [makeA, makeB, () => new C(), () => new D(), () => new M()]
       .map((f) => f())
       .toList();
   var a = things[0];
   var b = things[1];
   var c = things[2];
@@ skipping 38 matching lines @@
 
   Expect.equals("M-foo D.mm(cc)", d.foo());
   Expect.equals("D-bar -> C-baz cc", d.bar());
   Expect.equals("C-baz cc", d.baz());
   Expect.isFalse(d is A);
   Expect.isFalse(d is B);
   Expect.isTrue(d is C);
   Expect.isTrue(d is D);
   Expect.isTrue(d is M);
 }