Switch polymer's build.dart to use the new linter. This CL does the following

pkg/polymer/lib/src/file_system.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.
-
-/** Abstraction for file systems and utility functions to manipulate paths. */
-library file_system;
-
-import 'dart:async';
-
-/**
- * Abstraction around file system access to work in a variety of different
- * environments.
- */
-abstract class FileSystem {
-  /**
-   * Apply all pending writes.  Until this method is called, writeString is not
-   * guaranteed to have any observable impact.
-   */
-  Future flush();
-
-  /**
-   * Reads bytes if possible, but falls back to text if running in a browser.
-   * Return type is either [Future<List<int>>] or [Future<String>].
-   */
-  Future readTextOrBytes(String path);
-
-  /* Like [readTextOrBytes], but decodes bytes as UTF-8. Used for Dart code. */
-  Future<String> readText(String path);
-
-  /**
-   * Writes [text] to file at [path]. Call flush to insure that changes are
-   * visible.
-   */
-  void writeString(String path, String text);
-}