Move flat_buffer and api_signature into front_end.

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

Index: pkg/front_end/lib/src/base/api_signature.dart
diff --git a/pkg/analyzer/lib/src/summary/api_signature.dart b/pkg/front_end/lib/src/base/api_signature.dart
similarity index 98%
rename from pkg/analyzer/lib/src/summary/api_signature.dart
rename to pkg/front_end/lib/src/base/api_signature.dart
index d8c29c6c4d1adf93bcc4553f627481fa9922e0a9..f809dbb7ae7f37fb4c60836adb1f0809caffe041 100644
--- a/pkg/analyzer/lib/src/summary/api_signature.dart
+++ b/pkg/front_end/lib/src/base/api_signature.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.api_signature;
-
 import 'dart:convert';
 import 'dart:typed_data';