Fix dead code hints in summary code.

pkg/analyzer/lib/src/summary/builder.dart

 // Copyright (c) 2014, 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.
 
 /**
  * Format-agnostic infrastructure for building summary objects.
  */
 library analyzer.src.summary.builder;
 
-import 'dart:convert';
-
 /**
  * Instances of this class encapsulate the necessary state to keep track of a
  * serialized summary that is in the process of being built.
  *
  * This class is intended to be passed to the constructors of the summary
  * Builder classes.
  */
 class BuilderContext {
   // Note: at the moment this is a placeholder class since the current
   // serialization format (JSON) doesn't require any state tracking.  If/when
   // we switch to a serialization format that requires state tracking, the
   // state will be stored here.
 }