Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_field_invocation_test.dart

Index: tests/compiler/dart2js_native/native_field_invocation_test.dart
diff --git a/tests/compiler/dart2js_native/native_field_invocation_test.dart b/tests/compiler/dart2js_native/native_field_invocation_test.dart
index df6b9ab5365f28db3600493736cdf3fe2e74d05a..2bfb71dea88442c6c1908206d500850d2ea93027 100644
--- a/tests/compiler/dart2js_native/native_field_invocation_test.dart
+++ b/tests/compiler/dart2js_native/native_field_invocation_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";
 
 @Native("A")
 class A {
@@ -26,7 +25,7 @@ main() {
   b.foo = (x) => x + 1;
   b = nativeId(b); // Inferrer doesn't know if A has been instantiated.
   // At this point b could be A or B. The call to "foo" thus needs to go through
-  // an interceptor. Tests that the interceptor doesn't screw with retrieving
-  // the field and invoking the closure.
+  // an interceptor. Tests that the interceptor works for retrieving the Dart
+  // class field and invoking the closure.
   Expect.equals(499, b.foo(498));
 }