Fix ensure_build_test

Fixes #55

Author: kevmoo

json_serializable/test/ensure_build_test.dart

@@ -67,8 +67,10 @@ void main() {
     expect(_changedGeneratedFiles(), isEmpty);
 
     // 2 - run build - should be no output, since nothing should change
-    var result = _runProc('pub',
-        ['run', 'build_runner', 'build', '--delete-conflicting-outputs']);
+    var result =
+        _runProc('tool/build.dart', ['build', '--delete-conflicting-outputs']);
+
+    print(result);
     expect(result,
         contains(new RegExp(r'\[INFO\] Succeeded after \S+ with \d+ outputs')));