Move flat_buffer and api_signature into front_end.

pkg/front_end/lib/src/base/api_signature.dart

 // Copyright (c) 2016, 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 analyzer.src.summary.api_signature;
-
 import 'dart:convert';
 import 'dart:typed_data';
 
 import 'package:convert/convert.dart';
 import 'package:crypto/crypto.dart';
 
 /**
  * An instance of [ApiSignature] collects data in the form of primitive types
  * (strings, ints, bools, etc.) from a summary "builder" object, and uses them
  * to generate an MD5 signature of a the non-informative parts of the summary
@@ skipping 127 matching lines @@
     int oldLength = _data.lengthInBytes;
     if (_offset + spaceNeeded > oldLength) {
       int newLength = 2 * (_offset + spaceNeeded);
       ByteData newData = new ByteData(newLength);
       new Uint8List.view(newData.buffer)
           .setRange(0, oldLength, new Uint8List.view(_data.buffer));
       _data = newData;
     }
   }
 }