Add integration test: hot reload + incremental compiler

utils/kernel-service/kernel-service.dart

Index: utils/kernel-service/kernel-service.dart
diff --git a/utils/kernel-service/kernel-service.dart b/utils/kernel-service/kernel-service.dart
index 0e989dbba83602280c414e41e86ff5d1ef526ec2..957613bee19664e17fb17e0e295639227e5dc70a 100644
--- a/utils/kernel-service/kernel-service.dart
+++ b/utils/kernel-service/kernel-service.dart
@@ -29,6 +29,7 @@ import 'package:front_end/memory_file_system.dart';
 import 'package:front_end/physical_file_system.dart';
 import 'package:front_end/src/fasta/vm.dart'
     show CompilationResult, Status, parseScriptInFileSystem;
+import 'package:front_end/src/testing/hybrid_file_system.dart';
 
 const bool verbose = const bool.fromEnvironment('DFE_VERBOSE');
 
@@ -114,48 +115,6 @@ FileSystem _buildFileSystem(List namedSources) {
   return new HybridFileSystem(fileSystem);
 }
 
-/// A file system that mixes files from memory and a physical file system. All
-/// memory entities take priotity over file system entities.
-class HybridFileSystem implements FileSystem {
-  final MemoryFileSystem memory;
-  final PhysicalFileSystem physical = PhysicalFileSystem.instance;
-
-  HybridFileSystem(this.memory);
-
-  @override
-  FileSystemEntity entityForUri(Uri uri) => new HybridFileSystemEntity(
-      memory.entityForUri(uri), physical.entityForUri(uri));
-}
-
-/// Entity that delegates to an underlying memory or phisical file system
-/// entity.
-class HybridFileSystemEntity implements FileSystemEntity {
-  final FileSystemEntity memory;
-  final FileSystemEntity physical;
-
-  HybridFileSystemEntity(this.memory, this.physical);
-
-  FileSystemEntity _delegate;
-  Future<FileSystemEntity> get delegate async {
-    return _delegate ??= (await memory.exists()) ? memory : physical;
-  }
-
-  @override
-  Uri get uri => memory.uri;
-
-  @override
-  Future<bool> exists() async => (await delegate).exists();
-
-  @override
-  Future<DateTime> lastModified() async => (await delegate).lastModified();
-
-  @override
-  Future<List<int>> readAsBytes() async => (await delegate).readAsBytes();
-
-  @override
-  Future<String> readAsString() async => (await delegate).readAsString();
-}
-
 train(String scriptUri) {
   // TODO(28532): Enable on Windows.
   if (Platform.isWindows) return;