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

pkg/polymer/lib/src/custom_tag_name.dart

Index: pkg/polymer/lib/src/custom_tag_name.dart
diff --git a/pkg/polymer/lib/src/custom_tag_name.dart b/pkg/polymer/lib/src/custom_tag_name.dart
deleted file mode 100644
index 637b937dd13ed14068eb42ffd4652598775777e4..0000000000000000000000000000000000000000
--- a/pkg/polymer/lib/src/custom_tag_name.dart
+++ /dev/null
@@ -1,27 +0,0 @@
-// Copyright (c) 2013, 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 polymer.src.custom_tag_name;
-
-/**
- * Returns true if this is a valid custom element name. See:
- * <https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/custom/index.html#dfn-custom-element-name>
- */
-bool isCustomTag(String name) {
-  if (name == null || !name.contains('-')) return false;
-
-  // These names have meaning in SVG or MathML, so they aren't allowed as custom
-  // tags.
-  var invalidNames = const {
-    'annotation-xml': '',
-    'color-profile': '',
-    'font-face': '',
-    'font-face-src': '',
-    'font-face-uri': '',
-    'font-face-format': '',
-    'font-face-name': '',
-    'missing-glyph': '',
-  };
-  return !invalidNames.containsKey(name);
-}