Implement GC for FileState(s).

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 0211a42b668465923597b2282d8e0c58c15717e3..416f22ae151cf02021d1c738651fb6ae5fe2d564 100644
--- a/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
+++ b/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
@@ -79,8 +79,16 @@ class IncrementalKernelGeneratorImpl implements IncrementalKernelGenerator {
       : _logger = _options.logger,
         _byteStore = _options.byteStore {
     _computeSalt();
+
+    Future<Null> onFileAdded(Uri uri) {
+      if (watch != null) {
+        return watch(uri, true);
+      }
+      return new Future.value();
+    }
+
     _fsState = new FileSystemState(
-        _options.fileSystem, _uriTranslator, _salt, (uri) => watch(uri, true));
+        _options.fileSystem, _uriTranslator, _salt, onFileAdded);
   }
 
   @override
@@ -304,6 +312,9 @@ class IncrementalKernelGeneratorImpl implements IncrementalKernelGenerator {
           await file.refresh();
         }
       }
+
+      // The file graph might have changed, perform GC.
+      _fsState.gc(_entryPoint);
     });
   }