Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/static_methods_test.dart

Index: tests/compiler/dart2js_native/static_methods_test.dart
diff --git a/tests/compiler/dart2js_native/static_methods_test.dart b/tests/compiler/dart2js_native/static_methods_test.dart
index f1cfd44a3670d2ba2c53eab581b9c461194a2d4e..c646bfd80a5f28545e0fe86454b023346b779b6e 100644
--- a/tests/compiler/dart2js_native/static_methods_test.dart
+++ b/tests/compiler/dart2js_native/static_methods_test.dart
@@ -7,8 +7,8 @@
 //   identifier @JSName -> use @Native tag as 'scope' for @JSName.
 //   other @JSName -> use @JSName as an expression.
 
-import "package:expect/expect.dart";
-import 'dart:_js_helper' show Native, JSName, convertDartClosureToJS;
+import 'native_testing.dart';
+import 'dart:_js_helper' show convertDartClosureToJS;
 
 typedef int Callback(String s);
 
@@ -51,6 +51,7 @@ self.CC = CC;
 """;
 
 main() {
+  nativeTesting();
   setup();
 
   // TODO(sra): Investigate why this line is necessary to get a correctly