Upgrade some hints to errors (flutter#19)

These prevent building internally. Fix an unused import.

Author: natebosch

analysis_options.yaml

@@ -1,2 +1,8 @@
 analyzer:
   strong-mode: true
+  # These are errors when building in Google
+  errors:
+    unused_import: error
+    unused_element: error
+    unused_local_variable: error
+    dead_code: error

lib/src/json_document_transformer.dart

@@ -2,7 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:async';
 import 'dart:convert';
 
 import 'package:async/async.dart';