Revert "Add native_testing library to mock @Native classes"

tests/compiler/dart2js_native/native_field_invocation6_test.dart

 // Copyright (c) 2014, 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";
 
 makeA() native ;
 nativeFirst(x, y) native ;
 
 void setup() native """
 nativeFirst = function(x, y) { return x; }
 
 function A() {}
 makeA = function() { return new A; }
-self.nativeConstructor(A);
 """;
 
 @Native("A")
 class A {
   var _foo;
 
   get foo => _foo;
 
   init() {
     _foo = () => 42;
   }
 }
 
 class B {
   var foo = 499;
 }
 
 class C {
   foo() => 499;
 }
 
 class D {
   var foo = () => 499;
 }
 
 main() {
-  nativeTesting();
   setup();
   var a = makeA();
   a.init();
   var b = new B();
   var c = new C();
   var d = new D();
   a = nativeFirst(a, b);
   a = nativeFirst(a, c);
   a = nativeFirst(a, d);
   // The variable a is still an instance of class A, but dart2js can't infer
   // that anymore.
   Expect.equals(42, a.foo());
 }