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

tests/compiler/dart2js_native/downcast_test.dart

Index: tests/compiler/dart2js_native/downcast_test.dart
diff --git a/tests/compiler/dart2js_native/downcast_test.dart b/tests/compiler/dart2js_native/downcast_test.dart
index 44b689235d4c7223afc2afcde540422ead195cc4..0727b1ba47655e4428e020fd2d30bddc2d9ac897 100644
--- a/tests/compiler/dart2js_native/downcast_test.dart
+++ b/tests/compiler/dart2js_native/downcast_test.dart
@@ -28,8 +28,9 @@ 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;
@@ -45,12 +46,13 @@ 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 {}