Add a test for including all libraries from changed to the entry point.

pkg/front_end/lib/src/incremental_kernel_generator_impl.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/file_system.dart';
 import 'package:front_end/incremental_kernel_generator.dart';
 import 'package:front_end/src/base/api_signature.dart';
@@ skipping 108 matching lines @@
         for (Library library in result.kernelLibraries) {
           Uri uri = library.importUri;
           if (_latestSignature[uri] != result.signature) {
             _latestSignature[uri] = result.signature;
             program.libraries.add(library);
           }
         }
       }
 
       // TODO(scheglov) Add libraries which import changed libraries.
+      // For now the corresponding test works because we use full library
+      // contents to compute signatures (not just API parts). So, every library
+      // that imports a changed one, is affected.
 
       return new DeltaProgram(program);
     });
   }
 
   @override
   void invalidate(Uri uri) {
     _invalidatedFiles.add(uri);
   }
 
@@ skipping 160 matching lines @@
   /// TODO(scheglov) Use API signatures.
   /// TODO(scheglov) Or use tree shaking and compute signatures of outlines.
   final String signature;
 
   /// Kernel libraries for libraries in the [cycle].  Bodies of dependencies
   /// are not included, but but references to those dependencies are included.
   final List<Library> kernelLibraries;
 
   _LibraryCycleResult(this.cycle, this.signature, this.kernelLibraries);
 }