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 169 matching lines @@
     ..compileSdk = true // the incremental generator requires the sdk sources
     ..fileSystem = fs
     ..byteStore = new MemoryByteStore();
   return IncrementalKernelGenerator.newInstance(options, entryUri);
 }
 
 Future<bool> rebuild(IncrementalKernelGenerator compiler, Uri outputUri) async {
   compiler.invalidate(Uri.parse("file:///a.dart"));
   compiler.invalidate(Uri.parse("file:///b.dart"));
   var program = (await compiler.computeDelta()).newProgram;
+  compiler.acceptLastDelta();

[Siggi Cherem (dart-lang), 2017/08/10 18:31:15]

A couple suggestions: feel free to ignore if you don't want to bundle these
changes in this CL, I can follow up later with these as well.
 - might be nice to move this line to the hotReload instead (when we know that
the request was accepted)
 - we could add a test that we know will be rejected and then do a subsequent
reload with the new delta. We know that the VM will reject a reload if we
replace "class" by "enum", so we could add one such thing below.

[scheglov, 2017/08/10 19:28:39]

I moved this line into hotReload() and setUp().

I don't think there is a test for a failing hot reload, so I leave it without
tests.

   if (program != null && !program.libraries.isEmpty) {
     await writeProgram(program, outputUri);
     return true;
   }
   return false;
 }
 
 Future<Null> writeProgram(Program program, Uri outputUri) async {
   var sink = new File.fromUri(outputUri).openWrite();
   // TODO(sigmund): the incremental generator should always filter these
@@ skipping 39 matching lines @@
 
 f() => "$line part2";
 ''';
 
 const sourceB = r'''
 get line => "part1";
 ''';
 
 RegExp observatoryPortRegExp =
     new RegExp("Observatory listening on http://127.0.0.1:\([0-9]*\)/");