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

tests/compiler/dart2js_native/downcast_test.dart

 // Copyright (c) 2011, 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.
 
 // Test for downcasts on native classes.
 
 import "native_testing.dart";
 
 abstract class J {}
 
@@ skipping 10 matching lines @@
   A read() native;
   write(A x) native;
 }
 
 @Native("B")
 class B extends A {}
 
 makeA() native;
 makeB() native;
 
-void setup() native """
-// This code is all inside 'setup' and so not accessible from the global scope.
+void setup() {
+  JS('', r"""
+(function(){
 function inherits(child, parent) {
   if (child.prototype.__proto__) {
     child.prototype.__proto__ = parent.prototype;
   } else {
     function tmp() {};
     tmp.prototype = parent.prototype;
     child.prototype = new tmp();
     child.prototype.constructor = child;
   }
 }
 function A(){}
 function B(){}
 inherits(B, A);
 A.prototype.read = function() { return this._x; };
 A.prototype.write = function(x) { this._x = x; };
-makeA = function(){return new A};
-makeB = function(){return new B};
+makeA = function(){return new A()};
+makeB = function(){return new B()};
 
 self.nativeConstructor(A);
 self.nativeConstructor(B);
-""";
+})()""");
+}
 
 class C {}
 
 bool _check(a, b) => identical(a, b);
 
 main() {
   nativeTesting();
   setup();
 
   var a1 = makeA();
@@ skipping 11 matching lines @@
   Expect.equals(b1, b1 as I);
   Expect.equals(b1, b1 as A);
   Expect.equals(b1, b1 as B);
 
   Expect.equals(a1, a1 as J);
   Expect.equals(a1, a1 as I);
   Expect.equals(a1, a1 as A);
   Expect.throws(() => a1 as B);
   Expect.throws(() => a1 as C);
 }