Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_field_invocation6_test.dart

Index: tests/compiler/dart2js_native/native_field_invocation6_test.dart
diff --git a/tests/compiler/dart2js_native/native_field_invocation6_test.dart b/tests/compiler/dart2js_native/native_field_invocation6_test.dart
index 3d3b2640ff8a29d109e1235d8503187321501e78..ec27608be059d5e16fdad5b44636fa443173bfd8 100644
--- a/tests/compiler/dart2js_native/native_field_invocation6_test.dart
+++ b/tests/compiler/dart2js_native/native_field_invocation6_test.dart
@@ -2,8 +2,7 @@
 // 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";
 
 makeA() native ;
 nativeFirst(x, y) native ;
@@ -13,6 +12,7 @@ nativeFirst = function(x, y) { return x; }
 
 function A() {}
 makeA = function() { return new A; }
+self.nativeConstructor(A);
 """;
 
 @Native("A")
@@ -39,6 +39,7 @@ class D {
 }
 
 main() {
+  nativeTesting();
   setup();
   var a = makeA();
   a.init();