Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_mixin_multiple3_test.dart

 // Copyright (c) 2013, 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";
 
 // Test that native classes and plain classes can access methods defined only by
 // the same mixin.
 
 class D extends Object with M1, M2, M3 {}
 
 class E extends D {
   foo() => 'E.foo';
 }
 
@@ skipping 22 matching lines @@
 makeB() native ;
 makeC() native ;
 
 void setup() native """
 function A() {}
 function B() {}
 function C() {}
 makeA = function(){return new A;};
 makeB = function(){return new B;};
 makeC = function(){return new C;};
+
+self.nativeConstructor(A);
+self.nativeConstructor(B);
+self.nativeConstructor(C);
 """;
 
 var g;
 
 callFoo(x) {
   // Dominating getInterceptor call should be shared.
   g = x.toString();
   // These call sites are partitioned into pure-dart and native subsets,
   // allowing differences in getInterceptors.
   if (x is D) return x.foo();
   if (x is A) return x.foo();
 }
 
 makeAll() => [makeA(), makeB(), makeC(), new D(), new E()];
 
 main() {
+  nativeTesting();
   setup();
   /*
   var a = makeA();
   var b = makeB();
   var c = makeC();
   var d = new D();
   var e = new E();
   */
   var x = makeAll();
   var a = x[0];
   var b = x[1];
   var c = x[2];
   var d = x[3];
   var e = x[4];
 
   var f = callFoo;
 
   Expect.equals('A.foo', f(a));
   Expect.equals('M2.foo', f(b));
   Expect.equals('C.foo', f(c));
   Expect.equals('M2.foo', f(d));
   Expect.equals('E.foo', f(e));
 }