Implement promised watch behaviour for used == true.

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 b241aaba4252f0ca5739978001597faad5bedabd..0211a42b668465923597b2282d8e0c58c15717e3 100644
--- a/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
+++ b/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
@@ -74,16 +74,17 @@ class IncrementalKernelGeneratorImpl implements IncrementalKernelGenerator {
   final Set<Uri> _invalidatedFiles = new Set<Uri>();
 
   IncrementalKernelGeneratorImpl(
-      this._options, this._uriTranslator, this._entryPoint)
+      this._options, this._uriTranslator, this._entryPoint,
+      {WatchUsedFilesFn watch})
       : _logger = _options.logger,
         _byteStore = _options.byteStore {
     _computeSalt();
-    _fsState = new FileSystemState(_options.fileSystem, _uriTranslator, _salt);
+    _fsState = new FileSystemState(
+        _options.fileSystem, _uriTranslator, _salt, (uri) => watch(uri, true));

[Siggi Cherem (dart-lang), 2017/06/14 21:56:24]

Don't we need to check here that `watch` is not null? maybe pass a null callback
in that case?

Otherwise it appears like the check in filestate for _newFileFn != null will
always be true and we'll try to call the callback here, even when `watch` is
null.

Let's add a simple test to cover this too.

[scheglov, 2017/06/14 21:58:56]

You're right, this is a bug.
It was fixed in a next CL.
I will add a test.

[Siggi Cherem (dart-lang), 2017/06/14 22:25:00]

thanks - just saw the fix you landed later, thanks! I'm just going linearly in
time on my backlog of emails :)

   }
 
   @override
-  Future<DeltaProgram> computeDelta(
-      {Future<Null> watch(Uri uri, bool used)}) async {
+  Future<DeltaProgram> computeDelta() async {
     return await _logger.runAsync('Compute delta', () async {
       await _refreshInvalidatedFiles();
 
@@ -128,8 +129,6 @@ class IncrementalKernelGeneratorImpl implements IncrementalKernelGenerator {
         }
       }
 
-      if (watch != null) _fsState.fileUris.forEach((f) => watch(f, true));
-
       // TODO(scheglov) Add libraries which import changed libraries.
       // For now the corresponding test works because we use full library
       // contents to compute signatures (not just API parts). So, every library