Add IncrementalKernelGenerator.acceptLastDelta()/rejectLastDelta().

pkg/front_end/test/src/incremental/hot_reload_e2e_test.dart

Index: pkg/front_end/test/src/incremental/hot_reload_e2e_test.dart
diff --git a/pkg/front_end/test/src/incremental/hot_reload_e2e_test.dart b/pkg/front_end/test/src/incremental/hot_reload_e2e_test.dart
index 76dae64380ebbb06830924c567061f274a5bdb65..3c05b0d9668ff36514e24cbe6747ecade5524741 100644
--- a/pkg/front_end/test/src/incremental/hot_reload_e2e_test.dart
+++ b/pkg/front_end/test/src/incremental/hot_reload_e2e_test.dart
@@ -187,6 +187,7 @@ 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;