Move flat_buffer and api_signature into front_end.

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

Index: pkg/front_end/lib/src/base/flat_buffers.dart
diff --git a/pkg/analyzer/lib/src/summary/flat_buffers.dart b/pkg/front_end/lib/src/base/flat_buffers.dart
similarity index 99%
rename from pkg/analyzer/lib/src/summary/flat_buffers.dart
rename to pkg/front_end/lib/src/base/flat_buffers.dart
index 81bd2dbbb70a7a44dcb98a459b668bcd39f32833..6733ae113da4a58bc7fd68d5bddc6cfd29dff248 100644
--- a/pkg/analyzer/lib/src/summary/flat_buffers.dart
+++ b/pkg/front_end/lib/src/base/flat_buffers.dart
@@ -2,8 +2,6 @@
 // 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';