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

tests/compiler/dart2js_native/native_mixin_multiple3_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 plain classes can access methods defined only by
 // the same mixin.
 
 class D extends Object with M1, M2, M3 {}
@@ skipping 20 matching lines @@
 
 @Native("C")
 class C extends B {
   foo() => 'C.foo';
 }
 
 makeA() native;
 makeB() native;
 makeC() native;
 
-void setup() native """
-function A() {}
-function B() {}
-function C() {}
-makeA = function(){return new A;};
-makeB = function(){return new B;};
-makeC = function(){return new C;};
+void setup() {
+  JS('', r"""
+(function(){
+  function A() {}
+  function B() {}
+  function C() {}
+  makeA = function(){return new A()};
+  makeB = function(){return new B()};
+  makeC = function(){return new C()};
 
-self.nativeConstructor(A);
-self.nativeConstructor(B);
-self.nativeConstructor(C);
-""";
+  self.nativeConstructor(A);
+  self.nativeConstructor(B);
+  self.nativeConstructor(C);
+})()""");
+}
 
 var g;
 
 callFoo(x) {
   // Dominating getInterceptor call should be shared.
   g = x.toString();
   // These call sites are partitioned into pure-dart and native subsets,
   // allowing differences in getInterceptors.
   if (x is D) return x.foo();
   if (x is A) return x.foo();
@@ skipping 19 matching lines @@
   var e = x[4];
 
   var f = callFoo;
 
   Expect.equals('A.foo', f(a));
   Expect.equals('M2.foo', f(b));
   Expect.equals('C.foo', f(c));
   Expect.equals('M2.foo', f(d));
   Expect.equals('E.foo', f(e));
 }