Revert "Add native_testing library to mock @Native classes"

tests/compiler/dart2js_native/native_class_is_check3_frog_test.dart

 // Copyright (c) 2011, 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 "native_testing.dart";
+import "dart:_js_helper";
+import "package:expect/expect.dart";
 
 // Test for correct simple is-checks on hidden native classes.
 
 abstract class J {}
 
 abstract class I extends J {
   I read();
   write(I x);
 }
 
@@ skipping 24 matching lines @@
     child.prototype.constructor = child;
   }
 }
 function A(){}
 function B(){}
 inherits(B, A);
 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 {}
 
 main() {
-  nativeTesting();
   setup();
 
   var a1 = makeA();
   var b1 = makeB();
   var ob = new Object();
 
   Expect.isFalse(ob is J);
   Expect.isFalse(ob is I);
   Expect.isFalse(ob is A);
   Expect.isFalse(ob is B);
   Expect.isFalse(ob is C);
 
   // Use b1 first to prevent a1 is checks patching the A prototype.
   Expect.isTrue(b1 is J);
   Expect.isTrue(b1 is I);
   Expect.isTrue(b1 is A);
   Expect.isTrue(b1 is B);
   Expect.isTrue(b1 is! C);
 
   Expect.isTrue(a1 is J);
   Expect.isTrue(a1 is I);
   Expect.isTrue(a1 is A);
   Expect.isTrue(a1 is! B);
   Expect.isTrue(a1 is! C);
 }