Add IncrementalKernelGenerator.acceptLastDelta()/rejectLastDelta().

pkg/front_end/test/src/incremental/hot_reload_e2e_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.
 
 /// Integration test that runs the incremental compiler, runs the compiled
 /// program, incrementally rebuild portions of the app, and triggers a hot
 /// reload on the running program.
 library front_end.incremental.hot_reload_e2e_test;
 
 import 'dart:async';
@@ skipping 23 matching lines @@
   setUp(() async {
     outDir = Directory.systemTemp.createTempSync('hotreload_test');
     outputUri = outDir.uri.resolve('test.dill');
     fs = new MemoryFileSystem(Uri.parse('file:///'));
     writeFile(fs, 'a.dart', sourceA);
     writeFile(fs, 'b.dart', sourceB);
     writeFile(fs, '.packages', '');
     compiler = await createIncrementalCompiler(
         'file:///a.dart', new HybridFileSystem(fs));
     await rebuild(compiler, outputUri); // this is a full compile.
+    compiler.acceptLastDelta();
   });
 
   tearDown(() async {
     outDir.deleteSync(recursive: true);
     lines = null;
   });
 
   Future<int> computeVmPort() async {
     var portLine = await lines[0];
     expect(observatoryPortRegExp.hasMatch(portLine), isTrue);
@@ skipping 44 matching lines @@
     programIsDone = vm.exitCode;
     await resume();
   }
 
   /// Request a hot reload on the running program.
   Future hotReload() async {
     var port = await computeVmPort();
     var remoteVm = new RemoteVm(port);
     var reloadResult = await remoteVm.reload(outputUri);
     expect(reloadResult['success'], isTrue);
+    compiler.acceptLastDelta();
     await remoteVm.disconnect();
   }
 
   test('initial program is valid', () async {
     await startProgram(0);
     await programIsDone;
     expect(await lines[1], "part1 part2");
   });
 
   test('reload after leaf library modification', () async {
@@ skipping 121 matching lines @@
 
 f() => "$line part2";
 ''';
 
 const sourceB = r'''
 get line => "part1";
 ''';
 
 RegExp observatoryPortRegExp =
     new RegExp("Observatory listening on http://127.0.0.1:\([0-9]*\)/");