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

pkg/polymer/lib/src/files.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.
-
-library files;
-
-import 'package:html5lib/dom.dart';
-
-/** An input file to process by the template compiler. */
-class SourceFile {
-  static const int HTML = 1;
-  static const int DART = 2;
-  static const int STYLESHEET = 3;
-
-  final String path;
-  final int type;
-
-  Document document;
-
-  /** Dart code or contents of a linked style sheet. */
-  String code;
-
-  SourceFile(this.path, {this.type: HTML});
-
-  bool get isDart => type == DART;
-  bool get isHtml => type == HTML;
-  bool get isStyleSheet => type == STYLESHEET;
-
-  String toString() => "#<SourceFile $path>";
-}
-
-/** An output file to generated by the template compiler. */
-class OutputFile {
-  final String path;
-  final String contents;
-
-  /**
-   * Path to the source file that was transformed into this OutputFile, `null`
-   * for files that are generated and do not correspond to an input
-   * [SourceFile].
-   */
-  final String source;
-
-  OutputFile(this.path, this.contents, {this.source});
-
-  String toString() => "#<OutputFile $path>";
-}