Move flat_buffer and api_signature into front_end.

pkg/front_end/test/src/base/api_signature_test.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 test.src.summary.api_signature_test;
-
-import 'package:analyzer/src/summary/api_signature.dart';
 import 'package:analyzer/src/summary/format.dart';
 import 'package:convert/convert.dart';
 import 'package:crypto/crypto.dart';
+import 'package:front_end/src/base/api_signature.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(ApiSignatureTest);
   });
 }
 
 @reflectiveTest
@@ skipping 82 matching lines @@
     for (int i = 0; i < 100000; i++) {
       sig.addInt(i);
       expectedResult.add(i % 0x100);
       expectedResult.add((i ~/ 0x100) % 0x100);
       expectedResult.add((i ~/ 0x10000) % 0x100);
       expectedResult.add((i ~/ 0x1000000) % 0x100);
     }
     checkBytes(expectedResult);
   }
 }