Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_missing_method1_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';
 
 @Native("A")
 class A {}
 
-makeA() native ;
+A makeA() native ;
 
 void setup() native """
 function A() {};
-A.prototype.foo = function() { return  42; }
+A.prototype.foo = function() { return  99; }
 makeA = function() { return new A; }
+self.nativeConstructor(A);
 """;
 
 class B {
-  // We need to define a foo method so that Frog sees it. Because it's
-  // the only occurence of 'foo', Frog does not bother mangling the
-  // call sites. It thinks all calls will either go to this method, or
-  // throw a NoSuchMethodError.
+  // We need to define a foo method so that dart2js sees it. Because the the
+  // only occurences of 'foo' is on B, a Dart class, no interceptor is used.  It
+  // thinks all calls will either go to this method, or throw a
+  // NoSuchMethodError. It is possible that the native class will shadow a
+  // method, but it will not shadow 'foo' because the name is either 'mangled'
+  // with the arity, or minified.
   foo() {
     return 42;
   }
 }
 
 typedContext() {
-  var things = [makeA(), new B()];
-  A a = things[0];
+  confuse(new B()).foo();
+  A a = makeA();
   Expect.throws(() => a.foo(), (e) => e is NoSuchMethodError);
   Expect.throws(() => a.foo, (e) => e is NoSuchMethodError);
   Expect.throws(() => a.foo = 4, (e) => e is NoSuchMethodError);
 }
 
 untypedContext() {
-  var things = [makeA(), new B()];
-  var a = things[0];
+  confuse(new B()).foo();
+  var a = confuse(makeA());
   Expect.throws(() => a.foo(), (e) => e is NoSuchMethodError);
   Expect.throws(() => a.foo, (e) => e is NoSuchMethodError);
   Expect.throws(() => a.foo = 4, (e) => e is NoSuchMethodError);
 }
 
 main() {
+  nativeTesting();
   setup();
   typedContext();
   untypedContext();
 }