Add integration test: hot reload + incremental compiler

pkg/front_end/test/subpackage_relationships_test.dart

Index: pkg/front_end/test/subpackage_relationships_test.dart
diff --git a/pkg/front_end/test/subpackage_relationships_test.dart b/pkg/front_end/test/subpackage_relationships_test.dart
index 0f96056ad4e15088bf24f9bef5a811e425e623f4..3e493e3f9cef3953c0ca66f2610860d71c6869c5 100644
--- a/pkg/front_end/test/subpackage_relationships_test.dart
+++ b/pkg/front_end/test/subpackage_relationships_test.dart
@@ -135,6 +135,10 @@ final subpackageRules = {
     // 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.