Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_call_arity3_frog_test.dart

Index: tests/compiler/dart2js_native/native_call_arity3_frog_test.dart
diff --git a/tests/compiler/dart2js_native/native_call_arity3_frog_test.dart b/tests/compiler/dart2js_native/native_call_arity3_frog_test.dart
index 1b4048ec6c0fb1808a637e7b479b1b9884437ce3..f333d81716aa1bec29a6cabc97fa788e1203f818 100644
--- a/tests/compiler/dart2js_native/native_call_arity3_frog_test.dart
+++ b/tests/compiler/dart2js_native/native_call_arity3_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 similar to NativeCallArity1FrogTest, but with default values to
 // parameters set to null. These parameters should be treated as if they
@@ -34,12 +33,14 @@ 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));
@@ -78,6 +79,7 @@ testStaticContext() {
 }
 
 main() {
+  nativeTesting();
   setup();
   testDynamicContext();
   testStaticContext();