Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_call_arity1_frog_test.dart

Index: tests/compiler/dart2js_native/native_call_arity1_frog_test.dart
diff --git a/tests/compiler/dart2js_native/native_call_arity1_frog_test.dart b/tests/compiler/dart2js_native/native_call_arity1_frog_test.dart
index ed3d13153ac0513c5f2be3b65610a60035f9b308..d435bdd11ea2fc49bdd4e4d1286471a3617c54d7 100644
--- a/tests/compiler/dart2js_native/native_call_arity1_frog_test.dart
+++ b/tests/compiler/dart2js_native/native_call_arity1_frog_test.dart
@@ -2,8 +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";
+import "native_testing.dart";
 
 // Test that native methods with unnamed* optional arguments are called with the
 // number of arguments in the call site.  This is necessary because native
@@ -38,12 +37,14 @@ B.prototype.foo = function () { return arguments.length; };
 
 makeA = function(){return new A;};
 makeB = function(){return new B;};
+
+self.nativeConstructor(A);
+self.nativeConstructor(B);
 """;
 
 testDynamicContext() {
-  var things = [makeA(), makeB()];
-  var a = things[0];
-  var b = things[1];
+  var a = confuse(makeA());
+  var b = confuse(makeB());
 
   Expect.throws(() => a.foo());
   Expect.equals(1, a.foo(10));
@@ -76,6 +77,7 @@ testStaticContext() {
 }
 
 main() {
+  nativeTesting();
   setup();
   testDynamicContext();
   testStaticContext();