Fix disposing context with the new driver.

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

Index: pkg/analyzer/lib/src/dart/analysis/driver.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/driver.dart b/pkg/analyzer/lib/src/dart/analysis/driver.dart
index de4b10d88424d474566d6db9364bfcadc05b4887..81a95de9ac82a29631043cc499e056889357e262 100644
--- a/pkg/analyzer/lib/src/dart/analysis/driver.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/driver.dart
@@ -208,6 +208,16 @@ class AnalysisDriver {
   }
 
   /**
+   * Return the set of files added to analysis using [addFile].
+   */
+  Set<String> get addedFiles => _explicitFiles;
+
+  /**
+   * Return the set of files that are known, i.e. added or used implicitly.
+   */
+  Set<String> get knownFiles => _fsState.knownFiles;
+
+  /**
    * Set the list of files that the driver should try to analyze sooner.
    *
    * Every path in the list must be absolute and normalized.
@@ -356,14 +366,6 @@ class AnalysisDriver {
   }
 
   /**
-   * Return `true` if the file with the given [path] was explicitly added
-   * to analysis using [addFile].
-   */
-  bool isAddedFile(String path) {
-    return _explicitFiles.contains(path);
-  }
-
-  /**
    * Return the [Future] that completes with a [ParseResult] for the file
    * with the given [path].
    *