Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_call_arity3_frog_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // 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
 // do not have a default value for the native methods.
 
 @Native("A")
 class A {
   int foo(int x) native ;
 }
 
@@ skipping 10 matching lines @@
 
 void setup() native """
 function A() {}
 A.prototype.foo = function () { return arguments.length; };
 
 function B() {}
 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));
   Expect.throws(() => a.foo(10, 20));
   Expect.throws(() => a.foo(10, 20, 30));
 
   Expect.equals(0, b.foo());
   Expect.equals(1, b.foo(10));
   Expect.equals(2, b.foo(10, 20));
   Expect.equals(3, b.foo(10, 20, 30));
@@ skipping 18 matching lines @@
   Expect.equals(2, b.foo(10, 20));
   Expect.equals(3, b.foo(10, 20, 30));
 
   Expect.equals(1, b.foo(10));
   Expect.equals(2, b.foo(null, 20));
   Expect.equals(3, b.foo(null, null, 30));
   Expect.throws(() => b.foo(10, 20, 30, 40));
 }
 
 main() {
+  nativeTesting();
   setup();
   testDynamicContext();
   testStaticContext();
 }