Move some of analyzer's code generation utilities into front_end.

pkg/front_end/test/subpackage_relationships_test.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:front_end/compiler_options.dart';
 import 'package:front_end/dependency_grapher.dart';
 import 'package:path/path.dart' as pathos;
@@ skipping 13 matching lines @@
 /// TODO(paulberry): remove dependencies on analyzer.
 final subpackageRules = {
   'lib': new SubpackageRules(
       mayImportAnalyzer: true,
       allowedDependencies: ['lib/src', 'lib/src/base']),
   'lib/src': new SubpackageRules(
       mayImportAnalyzer: true,
       allowedDependencies: ['lib', 'lib/src/base', 'lib/src/scanner']),
   'lib/src/base': new SubpackageRules(
       mayImportAnalyzer: true, allowedDependencies: ['lib']),
+  'lib/src/codegen': new SubpackageRules(),
   'lib/src/fasta':
       new SubpackageRules(mayImportAnalyzer: false, allowedDependencies: [
     'lib/src/fasta/builder',
     'lib/src/fasta/dill',
     'lib/src/fasta/kernel',
     'lib/src/fasta/parser',
     'lib/src/fasta/scanner',
     'lib/src/fasta/testing',
     'lib/src/fasta/util',
   ]),
@@ skipping 190 matching lines @@
       problem('Uri $src is inside package:front_end but is not in any known '
           'subpackage');
     } else if (!subpackageRules[subpackage].allowSubdirs &&
         pathWithinSubpackage.contains('/')) {
       problem('Uri $src is in a subfolder of $subpackage, but that '
           'subpackage does not allow dart files in subdirectories.');
     }
     return subpackage;
   }
 }