(Reland) Start using FileState/FileSystemState to provide consistent view.

pkg/front_end/test/incremental_kernel_generator_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 'package:front_end/compiler_options.dart';
 import 'package:front_end/incremental_kernel_generator.dart';
 import 'package:front_end/memory_file_system.dart';
 import 'package:kernel/ast.dart';
@@ skipping 39 matching lines @@
     writeFile('/test/.packages', 'test:lib/');
     String path = '/test/lib/test.dart';
     Uri uri = writeFile(
         path,
         r'''
 main() {
   var v = 1;
 }
 ''');
 
-    // Compute the initial state
-    {
-      Program program = await getInitialState(uri);
-      Library library = _getLibrary(program, uri);
-      expect(
-          _getLibraryText(library),
-          r'''
+    String initialText = r'''
 library;
 import self as self;
 import "dart:core" as core;
 
 static method main() → dynamic {
   core::int v = 1;
 }
-''');
+''';
+
+    // Compute the initial state.
+    {
+      Program program = await getInitialState(uri);
+      Library library = _getLibrary(program, uri);
+      expect(_getLibraryText(library), initialText);
     }
 
     // Update the entry point library.
     writeFile(
         path,
         r'''
 main() {
   var v = 2.3;
 }
 ''');
 
-    // The delta has the updated entry point library.
+    // Because we have not invalidated the file, we get the same library.
+    // TODO(scheglov) Eventually we should get an empty Program.
     {
       DeltaProgram delta = await incrementalKernelGenerator.computeDelta();
-      Program program = delta.newProgram;
-      Library library = _getLibrary(program, uri);
+      Library library = _getLibrary(delta.newProgram, uri);
+      expect(_getLibraryText(library), initialText);
+    }
+
+    // Invalidate the file, so get the new text.
+    incrementalKernelGenerator.invalidate(uri);
+    {
+      DeltaProgram delta = await incrementalKernelGenerator.computeDelta();
+      Library library = _getLibrary(delta.newProgram, uri);
       expect(
           _getLibraryText(library),
           r'''
 library;
 import self as self;
 import "dart:core" as core;
 
 static method main() → dynamic {
   core::double v = 2.3;
 }
@@ skipping 42 matching lines @@
     throw fail('No library found with URI "$uri"');
   }
 
   String _getLibraryText(Library library) {
     StringBuffer buffer = new StringBuffer();
     new Printer(buffer, syntheticNames: new NameSystem())
         .writeLibraryFile(library);
     return buffer.toString();
   }
 }