Add native_testing library to mock @Native classes

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 f993388d1094b1a73a080d15c46a74dcf0a6b216..dab6ec8901439c9fc7234511cbb64bd3fb21817b 100644
--- a/tests/compiler/dart2js_native/downcast_test.dart
+++ b/tests/compiler/dart2js_native/downcast_test.dart
@@ -4,8 +4,7 @@
 
 // Test for downcasts on native classes.
 
-import "dart:_js_helper";
-import "package:expect/expect.dart";
+import "native_testing.dart";
 
 abstract class J {}
 
@@ -48,6 +47,9 @@ 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};
+
+self.nativeConstructor(A);
+self.nativeConstructor(B);
 """;
 
 class C {}
@@ -55,6 +57,7 @@ class C {}
 bool _check(a, b) => identical(a, b);
 
 main() {
+  nativeTesting();
   setup();
 
   var a1 = makeA();