Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/field_type_test.dart

Index: tests/compiler/dart2js_native/field_type_test.dart
diff --git a/tests/compiler/dart2js_native/field_type_test.dart b/tests/compiler/dart2js_native/field_type_test.dart
index ebb83c66f106b8ed0eb22fb728a0af7fb63284c9..c9f4ec999d7006e7b245bdcb88d0610dbf9ff4ec 100644
--- a/tests/compiler/dart2js_native/field_type_test.dart
+++ b/tests/compiler/dart2js_native/field_type_test.dart
@@ -6,8 +6,7 @@
 // This regression test verifies that compiler accounts for hidden constructor
 // when analysing field values.
 
-import "dart:_js_helper";
-import "package:expect/expect.dart";
+import "native_testing.dart";
 
 @Native("Node")
 class Node {
@@ -48,9 +47,11 @@ void setup() native """
 // This code is all inside 'setup' and so not accesible from the global scope.
 function Node(parent){ this.parentNode = parent; }
 makeNode = function(p){return new Node(p);};
+self.nativeConstructor(Node);
 """;
 
 main() {
+  nativeTesting();
   setup();
 
   var n1 = makeNode(null);