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;