pkg/compiler: fix imports

pkg/compiler/lib/src/serialization/json_serializer.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 library dart2js.serialization.json;
 
 import 'dart:convert';
+
 import 'keys.dart';
 import 'serialization.dart';
 import 'values.dart';
 
 /// Serialization encoder for JSON.
 class JsonSerializationEncoder implements SerializationEncoder {
   const JsonSerializationEncoder();
 
   String encode(ObjectValue objectValue) {
     return new JsonEncoder.withIndent(' ')
@@ skipping 186 matching lines @@
   @override
   void visitType(TypeValue value, String indentation) {
     buffer.write('Type(${value.id}):${value.type}');
   }
 
   @override
   void visitUri(UriValue value, String indentation) {
     buffer.write('Uri(${value.value})');
   }
 }