Move flat_buffer and api_signature into front_end.

pkg/front_end/lib/src/base/flat_buffers.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.flat_buffers;
-
 import 'dart:collection';
 import 'dart:convert';
 import 'dart:math';
 import 'dart:typed_data';
 
 /**
  * Reader of lists of boolean values.
  *
  * The returned unmodifiable lists lazily read values on access.
  */
@@ skipping 931 matching lines @@
     // Table size.
     buf.setUint16(bufOffset, tableSize, Endianness.LITTLE_ENDIAN);
     bufOffset += 2;
     // Field offsets.
     for (int fieldOffset in fieldOffsets) {
       buf.setUint16(bufOffset, fieldOffset, Endianness.LITTLE_ENDIAN);
       bufOffset += 2;
     }
   }
 }