Move flat_buffer and api_signature into front_end.

pkg/front_end/test/src/base/flat_buffers_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.flat_buffers_test;
-
 import 'dart:typed_data';
 
-import 'package:analyzer/src/summary/flat_buffers.dart';
+import 'package:front_end/src/base/flat_buffers.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(BuilderTest);
   });
 }
 
 @reflectiveTest
@@ skipping 400 matching lines @@
 }
 
 class TestPointReader extends TableReader<TestPointImpl> {
   const TestPointReader();
 
   @override
   TestPointImpl createObject(BufferContext object, int offset) {
     return new TestPointImpl(object, offset);
   }
 }