Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/type_error_decode_test.dart

Index: tests/compiler/dart2js_native/type_error_decode_test.dart
diff --git a/tests/compiler/dart2js_native/type_error_decode_test.dart b/tests/compiler/dart2js_native/type_error_decode_test.dart
index 3c9a8f0ace7c5a0d482559c2113ec25c52208170..1c37f03964a09acf09893036e71850a8f8c6b293 100644
--- a/tests/compiler/dart2js_native/type_error_decode_test.dart
+++ b/tests/compiler/dart2js_native/type_error_decode_test.dart
@@ -4,9 +4,8 @@
 
 library test.type_error_decode_test;
 
-import 'package:expect/expect.dart';
-
-import 'dart:_js_helper';
+import 'native_testing.dart';
+import 'dart:_js_helper' show NullError, JsNoSuchMethodError;
 
 class Foo {
   var field;
@@ -41,12 +40,19 @@ main() {
   var s = "Cannot call method 'foo' of null";
   var nul = null;
   var f = new Foo();
-  // This should foil code analysis so the variables aren't inlined below.
-  [].forEach((y) => f.field = nul = s = x = z = v = y);
+
   expectThrows(() => x.fisk(), isNullError);
   expectThrows(() => v.fisk(), isNullError);
   expectThrows(() => z.fisk(), isJsNoSuchMethodError);
   expectThrows(() => s.fisk(), isJsNoSuchMethodError);
   expectThrows(() => null(), isNullError);
   expectThrows(() => f.field(), isNullError);
+
+  expectThrows(() => confuse(x).fisk(), isNullError);
+  expectThrows(() => confuse(v).fisk(), isNullError);
+  expectThrows(() => confuse(z).fisk(), isJsNoSuchMethodError);
+  expectThrows(() => confuse(s).fisk(), isJsNoSuchMethodError);
+  expectThrows(() => confuse(null)(), isNullError);
+  expectThrows(() => confuse(f).field(), isNullError);
+  expectThrows(() => confuse(f.field)(), isNullError);
 }