Remove unused imports in pub.

sdk/lib/_internal/pub/lib/src/directory_tree.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// A simple library for rendering a list of files as a directory tree.
 library pub.directory_tree;
 
 import 'package:path/path.dart' as path;
 
-import 'log.dart' as log;
-
 /// Draws a directory tree for the given list of files. Given a list of files
 /// like:
 ///
 ///     TODO
 ///     example/console_example.dart
 ///     example/main.dart
 ///     example/web copy/web_example.dart
 ///     test/absolute_test.dart
 ///     test/basename_test.dart
 ///     test/dirname_test.dart
@@ skipping 104 matching lines @@
     buffer.write(prefix);
     buffer.write(_getPrefix(name == null, isLast));
     buffer.writeln('| (${childNames.length - 6} more...)');
 
     // Show the last few.
     _drawChild(false, childNames[childNames.length - 3]);
     _drawChild(false, childNames[childNames.length - 2]);
     _drawChild(true, childNames[childNames.length - 1]);
   }
 }