Add integration test: hot reload + incremental compiler

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 117 matching lines @@
     'lib/src/fasta/parser',
     'lib/src/fasta/source',
   ]),
   'lib/src/scanner': new SubpackageRules(allowedDependencies: [
     'lib/src/base',
     // For error codes.
     'lib/src/fasta',
     // fasta scanner produces analyzer scanner tokens
     'lib/src/fasta/scanner',
   ]),
+  'lib/src/testing': new SubpackageRules(allowedDependencies: [
+    'lib',
+  ]),
+  'lib/src/vm': new SubpackageRules(allowedDependencies: []),
 };
 
 /// Rules for what a subpackage may depend directly on.
 class SubpackageRules {
   /// Indicates whether the subpackage may directly depend on analyzer.
   final bool mayImportAnalyzer;
 
   /// Indicates whether dart files may exist in subdirectories of this
   /// subpackage.
   ///
@@ skipping 145 matching lines @@
       if (subpackageRules[subpackage].allowSubdirs) {
         subpackageRules[subpackage].actuallyHasSubdirs = true;
       } else {
         problem('Uri $src is in a subfolder of $subpackage, but that '
             'subpackage does not allow dart files in subdirectories.');
       }
     }
     return subpackage;
   }
 }