Don't do folder existence checks when parsing pub list results.

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 a53f0e6c069466dc6ebd357bc3dedb971fa1c953..5f32f816722e313d135751e4ff5dbd79e600665f 100644
--- a/pkg/analyzer/lib/file_system/file_system.dart
+++ b/pkg/analyzer/lib/file_system/file_system.dart
@@ -77,6 +77,14 @@ abstract class Folder extends Resource {
    * in this folder, in no particular order.
    */
   List<Resource> getChildren();
+
+  /**
+   * Return a [Folder] representing a child [Resource] with the given
+   * [relPath].  This call does not check whether a folder with the given name
+   * exists on the filesystem--client must call the [Folder]'s `exists` getter
+   * to determine whether the folder actually exists.
+   */
+  Folder getChildAssumingFolder(String relPath);
 }