Remove old frog-style dummy bodies from dart2js native class tests

tests/compiler/dart2js_native/native_checked_fields_frog_test.dart

Index: tests/compiler/dart2js_native/native_checked_fields_frog_test.dart
diff --git a/tests/compiler/dart2js_native/native_checked_fields_frog_test.dart b/tests/compiler/dart2js_native/native_checked_fields_frog_test.dart
index 7648da46102d2763b64d865bb5e432301afd6f9a..8992dc0790c560509610045f09e918a9d0dd1ba6 100644
--- a/tests/compiler/dart2js_native/native_checked_fields_frog_test.dart
+++ b/tests/compiler/dart2js_native/native_checked_fields_frog_test.dart
@@ -17,8 +17,8 @@ class B {
   String foo;
 }
 
-A makeA() native { return new A(); }
-B makeB() native { return new B(); }
+A makeA() native;
+B makeB() native;
 
 void setup() native """
 function A() {}