Move flat_buffer and api_signature into front_end.

pkg/analyzer/lib/src/summary/summarize_elements.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 serialization.elements;
 
 import 'dart:convert';
 
 import 'package:analyzer/src/generated/source.dart';
-import 'package:analyzer/src/summary/api_signature.dart';
 import 'package:analyzer/src/summary/format.dart';
 import 'package:analyzer/src/summary/idl.dart';
 import 'package:analyzer/src/summary/package_bundle_reader.dart';
 import 'package:convert/convert.dart';
 import 'package:crypto/crypto.dart';
+import 'package:front_end/src/base/api_signature.dart';
 
 /**
  * Object that gathers information uses it to assemble a new
  * [PackageBundleBuilder].
  */
 class PackageBundleAssembler {
   /**
    * Value that will be stored in [PackageBundle.majorVersion] for any summaries
    * created by this code.  When making a breaking change to the summary format,
    * this value should be incremented by 1 and [currentMinorVersion] should be
@@ skipping 83 matching lines @@
     return apiSignature.toHex();
   }
 
   /**
    * Compute a hash of the given file contents.
    */
   String _hash(String contents) {
     return hex.encode(md5.convert(UTF8.encode(contents)).bytes);
   }
 }