Package map source factory support [FIXED].

pkg/analyzer/lib/file_system/file_system.dart

Index: pkg/analyzer/lib/file_system/file_system.dart
diff --git a/pkg/analyzer/lib/file_system/file_system.dart b/pkg/analyzer/lib/file_system/file_system.dart
index 62c158b12447cd8021f4da0ea0ec1547e2c7b473..899d6c5bf6227ad7b2ff4b2e8f349f96e0913e88 100644
--- a/pkg/analyzer/lib/file_system/file_system.dart
+++ b/pkg/analyzer/lib/file_system/file_system.dart
@@ -139,6 +139,20 @@ abstract class ResourceProvider {
   Context get pathContext;
 
   /**
+   * Return a [File] that corresponds to the given [path].
+   * 
+   * A file may or may not exist at this location.
+   */
+  File getFile(String path);
+
+  /**
+   * Return a [Folder] that corresponds to the given [path].
+   * 
+   * A folder may or may not exist at this location.
+   */
+  Folder getFolder(String path);
+
+  /**
    * Return the [Resource] that corresponds to the given [path].
    */
   Resource getResource(String path);