Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/core_type_check_native_test.dart

Index: tests/compiler/dart2js_native/core_type_check_native_test.dart
diff --git a/tests/compiler/dart2js_native/core_type_check_native_test.dart b/tests/compiler/dart2js_native/core_type_check_native_test.dart
index 261acf7f757db901afa2036b61c4d44218d4e9de..307509f5f9d55ff85fbf2b40d40cf77dc098685e 100644
--- a/tests/compiler/dart2js_native/core_type_check_native_test.dart
+++ b/tests/compiler/dart2js_native/core_type_check_native_test.dart
@@ -2,10 +2,7 @@
 // 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 "dart:_js_helper";
-import "package:expect/expect.dart";
-
-var inscrutable = (int x) => x == 0 ? 0 : x | inscrutable(x & (x - 1));
+import "native_testing.dart";
 
 @Native("A")
 class A {}
@@ -33,6 +30,11 @@ function C() {};
 makeC = function() { return new C; }
 function D() {};
 makeD = function() { return new D; }
+
+self.nativeConstructor(A);
+self.nativeConstructor(B);
+self.nativeConstructor(C);
+self.nativeConstructor(D);
 """;
 
 checkTest(value, expectComparable, expectPattern) {
@@ -65,7 +67,7 @@ checkAll(check) {
     makeC(),
     makeD()
   ];
-  value(i) => things[inscrutable(i)];
+  value(i) => confuse(things[i]);
 
   check(value(0), false, false); // List
   check(value(1), true, false); // int
@@ -79,6 +81,7 @@ checkAll(check) {
 }
 
 main() {
+  nativeTesting();
   setup();
 
   checkAll(checkTest);