Add IncrementalKernelGenerator.acceptLastDelta()/rejectLastDelta().

pkg/front_end/lib/src/incremental_kernel_generator_impl.dart

Index: pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
diff --git a/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart b/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
index 025184ee65af91fa4c7666a482656d906a14428b..77cd524f5212305080fb4ea12fa7c1627e3e0b83 100644
--- a/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
+++ b/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
@@ -36,8 +36,12 @@ class IncrementalKernelGeneratorImpl implements IncrementalKernelGenerator {
   /// The [KernelDriver] that is used to compute kernels.
   KernelDriver _driver;
 
-  /// Latest compilation signatures produced by [computeDelta] for libraries.
-  final Map<Uri, String> _latestSignature = {};
+  /// The current signatures for libraries.
+  final Map<Uri, String> _currentSignatures = {};
+
+  /// The signatures for libraries produced by the last [computeDelta], or
+  /// `null` if the last delta was either accepted or rejected.
+  Map<Uri, String> _lastSignatures;
 
   /// The object that provides additional information for tests.
   _TestView _testView;
@@ -64,8 +68,21 @@ class IncrementalKernelGeneratorImpl implements IncrementalKernelGenerator {
   @visibleForTesting
   _TestView get test => _testView;
 
+  @override
+  void acceptLastDelta() {
+    _throwIfNoLastDelta();
+    _currentSignatures.addAll(_lastSignatures);
+    _lastSignatures = null;
+  }
+
   @override
   Future<DeltaProgram> computeDelta() async {
+    if (_lastSignatures != null) {
+      throw new StateError(

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

question: instead of throwing, should we instead report an error message via
options.reportError? (and define an error in messages.yaml)?

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

I don't know.

Outside of front_end I would use StateError, because nothing good is going to
happen when you use API incorrectly.
It is not a compilation error, or a transient problem - it is a programmer
error.

+          'The last delta must be either accepted or rejected.');
+    }
+    _lastSignatures = {};

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

it might be good to also enforce that accept/reject is not called until after
computeDelta() completes. 
Two ideas for that:
- we could keep this as a local variable and only assign it as a field when you
are about to return the DeltaProgram
- add a second field to track that there is a computeDelta in progress.

Also, since you said that calling `invalidate` is invalid if there is a pending
computeDelta, we might want to do that too. If so, I guess the latter idea might
make more sense.

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

This is not a new contract, I will return to enforcement in a new CL.

+
     return await _logger.runAsync('Compute delta', () async {
       KernelResult kernelResult = await _driver.getKernel(_entryPoint);
       List<LibraryCycleResult> results = kernelResult.results;
@@ -78,8 +95,8 @@ class IncrementalKernelGeneratorImpl implements IncrementalKernelGenerator {
       for (LibraryCycleResult result in results) {
         for (Library library in result.kernelLibraries) {
           Uri uri = library.importUri;
-          if (_latestSignature[uri] != result.signature) {
-            _latestSignature[uri] = result.signature;
+          if (_currentSignatures[uri] != result.signature) {
+            _lastSignatures[uri] = result.signature;
             affectedLibraryCycles.add(result.cycle);
           }
         }
@@ -130,6 +147,12 @@ class IncrementalKernelGeneratorImpl implements IncrementalKernelGenerator {
     _driver.invalidate(uri);
   }
 
+  @override
+  void rejectLastDelta() {
+    _throwIfNoLastDelta();
+    _lastSignatures = null;
+  }
+
   /// TODO(scheglov) document
   Future<Null> _gc() async {
     var removedFiles = _driver.fsState.gc(_entryPoint);
@@ -139,6 +162,16 @@ class IncrementalKernelGeneratorImpl implements IncrementalKernelGenerator {
       }
     }
   }
+
+  /// Throw [StateError] if [_lastSignatures] is `null`, i.e. there is no
+  /// last delta - it either has not been computed yet, or has been already
+  /// accepted or rejected.
+  void _throwIfNoLastDelta() {
+    if (_lastSignatures == null) {
+      throw new StateError(
+          'The last delta has been already accepted or rejected.');

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

same question: report or throw?

+    }
+  }
 }
 
 @visibleForTesting