Fix disposing context with the new driver.

pkg/analyzer/lib/src/dart/analysis/file_state.dart

Index: pkg/analyzer/lib/src/dart/analysis/file_state.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/file_state.dart b/pkg/analyzer/lib/src/dart/analysis/file_state.dart
index cbca17a9fa19ef2f2a73b20a774f2d043e820c5d..89a3d7b374ab01e84f226717dfbe2431991f3695 100644
--- a/pkg/analyzer/lib/src/dart/analysis/file_state.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/file_state.dart
@@ -378,6 +378,11 @@ class FileSystemState {
       this._salt);
 
   /**
+   * Return the set of known files.
+   */
+  Set<String> get knownFiles => _pathToFiles.keys.toSet();
+
+  /**
    * Return the canonical [FileState] for the given absolute [path]. The
    * returned file has the last known state since if was last refreshed.
    *