remove old custom_element polyfill

pkg/custom_element/lib/src/attribute_map.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.
-
-part of custom_element;
-
-/**
- * Represents an attribute map of model values. If any items are added,
- * removed, or replaced, then observers that are listening to [changes]
- * will be notified.
- */
-class _AttributeMap implements Map<String, String> {
-  final CustomElement _element;
-  final Map<String, String> _map;
-
-  /** Creates an attribute map wrapping the host attributes. */
-  _AttributeMap(CustomElement element)
-      : _element = element, _map = element.host.attributes;
-
-  // Forward all read methods:
-  Iterable<String> get keys => _map.keys;
-  Iterable<String> get values => _map.values;
-  int get length =>_map.length;
-  bool get isEmpty => _map.isEmpty;
-  bool get isNotEmpty => _map.isNotEmpty;
-  bool containsValue(Object value) => _map.containsValue(value);
-  bool containsKey(Object key) => _map.containsKey(key);
-  String operator [](Object key) => _map[key];
-  void forEach(void f(String key, String value)) => _map.forEach(f);
-  String toString() => _map.toString();
-
-  // Override the write methods and ensure attributeChanged is called:
-  void operator []=(String key, String value) {
-    int len = _map.length;
-    String oldValue = _map[key];
-    _map[key] = value;
-    if (len != _map.length || !identical(oldValue, value)) {
-      _element.attributeChanged(key, oldValue);
-    }
-  }
-
-  void addAll(Map<String, String> other) {
-    other.forEach((String key, String value) { this[key] = value; });
-  }
-
-  String putIfAbsent(String key, String ifAbsent()) {
-    int len = _map.length;
-    String result = _map.putIfAbsent(key, ifAbsent);
-    if (len != _map.length) {
-      _element.attributeChanged(key, null);
-    }
-    return result;
-  }
-
-  String remove(Object key) {
-    int len = _map.length;
-    String result =  _map.remove(key);
-    if (len != _map.length) {
-      _element.attributeChanged(key, result);
-    }
-    return result;
-  }
-
-  void clear() {
-    int len = _map.length;
-    if (len > 0) {
-      _map.forEach((key, value) {
-        _element.attributeChanged(key, value);
-      });
-    }
-    _map.clear();
-  }
-
-  /**
-   * This is not a [Map] method. We use it to implement "set attributes", which
-   * is a global replace operation. Rather than [clear] followed by [addAll],
-   * we try to be a bit smarter.
-   */
-  void _replaceAll(Map<String, String> other) {
-    for (var key in keys) {
-      if (!other.containsKey(key)) remove(key);
-    }
-    addAll(other);
-  }
-}