Revert "Add native_testing library to mock @Native classes"

tests/compiler/dart2js_native/native_testing.dart

-// Copyright (c) 2016, 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.
-
-// Native testing library. Provides support for mock @Native classes and
-// collects common imports.
-
-
-import "package:expect/expect.dart";
-import 'dart:_js_helper' show Native;
-
-export "package:expect/expect.dart";
-export 'dart:_js_helper' show Creates, Native, JSName, Returns;
-export 'dart:_foreign_helper' show JS;
-
-void _setup() native r'''
-((function() {
-  var toStringResultProperty = "_toStringResult";
-  var objectToStringMethod = Object.prototype.toString;
-
-  Object.prototype.toString = function() {
-    if (this != null) {
-      var constructor = this.constructor;
-      if (constructor != null) {
-        var result = constructor[toStringResultProperty];
-        if (typeof result == "string") return result;
-      }
-    }
-    return objectToStringMethod.call(this);
-  }
-
-  // To mock a @Native class with JavaScript constructor `Foo`, add
-  //
-  //     self.nativeConstructor(Foo);
-  //
-  // to the JavaScript code.
-  self.nativeConstructor = function(constructor, opt_name) {
-    var toStringResult = "[object " + (opt_name || constructor.name) + "]";
-    constructor[toStringResultProperty] = toStringResult;
-  }
-})());
-''';
-
-void nativeTesting() {
-  _setup();
-}
-
-@NoInline()
-@AssumeDynamic()
-confuse(x) => x;