add custom_element package

pkg/custom_element/lib/src/custom_tag_name.dart

+// 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 custom_element.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.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);
+}