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

tests/compiler/dart2js_native/native_mixin_field_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 use ordinary Dart classes with fields
 // as mixins.
 
 @Native("A")
@@ skipping 11 matching lines @@
 }
 
 class M2 {
   var bar;
   var buz;
 }
 
 A makeA() native;
 B makeB() native;
 
-void setup() native """
-function A() {this.foo='A-foo';}
-function B() {A.call(this);this.bar='B-bar';this.baz='M1-baz';}
-makeA = function(){return new A;};
-makeB = function(){return new B;};
+void setup() {
+  JS('', r"""
+(function(){
+  function A() {this.foo='A-foo';}
+  function B() {A.call(this);this.bar='B-bar';this.baz='M1-baz';}
+  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();
   A a = makeA();
   Expect.equals("A-foo", a.foo);
   Expect.throws(() => a.bar, (e) => e is NoSuchMethodError);
   Expect.throws(() => a.baz, (e) => e is NoSuchMethodError);
   Expect.throws(() => a.buz, (e) => e is NoSuchMethodError);
 
   B b = makeB();
   Expect.equals("A-foo", b.foo);
   Expect.equals("B-bar", b.bar);
   // Expect.equals("M1-baz", b.baz);  // not true, see M1.
   Expect.isNull(b.baz); // native b.baz is not the same as dart b.baz.
   Expect.isNull(b.buz);
 
   M1 m1 = new M1();
   Expect.throws(() => m1.foo, (e) => e is NoSuchMethodError);
   Expect.throws(() => m1.bar, (e) => e is NoSuchMethodError);
   Expect.isNull(m1.baz);
   Expect.throws(() => m1.buz, (e) => e is NoSuchMethodError);
 
   M2 m2 = new M2();
   Expect.throws(() => m2.foo, (e) => e is NoSuchMethodError);
   Expect.isNull(m2.bar);
   Expect.throws(() => m2.baz, (e) => e is NoSuchMethodError);
   Expect.isNull(m2.buz);
 }