Add IncrementalKernelGenerator.acceptLastDelta()/rejectLastDelta().

pkg/front_end/example/incremental_reload/compiler_with_invalidation.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.
 
 /// A wrapper on top of the [IncrementalKernelGenerator] that tracks
 /// file modifications between subsequent compilation requests and only
 /// invalidates those files that appear to be modified.
 library front_end.example.incremental_reload.compiler_with_invalidation;
 
 import 'dart:io';
@@ skipping 89 matching lines @@
         _generator.invalidate(uri);
         changed++;
       }
     }
     invalidateTimer.stop();
     invalidateTime = invalidateTimer.elapsedMilliseconds;
     if (changed == 0 && lastModified.isNotEmpty) return null;
 
     var compileTimer = new Stopwatch()..start();
     var delta = await _generator.computeDelta();
+    _generator.acceptLastDelta();
     compileTimer.stop();
     compileTime = compileTimer.elapsedMilliseconds;
     var program = delta.newProgram;
     return program;
   }
 }
 
 /// The result of an incremental compile and metrics collected during the
 /// the compilation.
 class CompilationResult {
@@ skipping 42 matching lines @@
     result.errorSeen = true;
   }
 
   result.changed = compiler.changed;
   result.totalFiles = compiler.lastModified.length;
   result.invalidateTime = compiler.invalidateTime;
   result.compileTime = compiler.compileTime;
   result.reloadTime = 0;
   return result;
 }