// 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.

part of html;

// TODO(jacobr): use _Lists.dart to remove some of the duplicated
// functionality.
class _ChildrenElementList implements List {
  // Raw Element.
  final Element _element;
  final HtmlCollection _childElements;

  _ChildrenElementList._wrap(Element element)
    : _childElements = element.$dom_children,
      _element = element;

  List<Element> toList() {
    final output = new List<Element>.fixedLength(_childElements.length);
    for (int i = 0, len = _childElements.length; i < len; i++) {
      output[i] = _childElements[i];
    }
    return output;
  }

  Set<Element> toSet() {
    final output = new Set<Element>();
    for (int i = 0, len = _childElements.length; i < len; i++) {
      output.add(_childElements[i]);
    }
    return output;
  }

  bool contains(Element element) => _childElements.contains(element);

  void forEach(void f(Element element)) {
    for (Element element in _childElements) {
      f(element);
    }
  }

  bool every(bool f(Element element)) {
    for (Element element in this) {
      if (!f(element)) {
        return false;
      }
    }
    return true;
  }

  bool any(bool f(Element element)) {
    for (Element element in this) {
      if (f(element)) {
        return true;
      }
    }
    return false;
  }

  String join([String separator]) {
    return IterableMixinWorkaround.joinList(this, separator);
  }

  List mappedBy(f(Element element)) {
    return IterableMixinWorkaround.mappedByList(this, f);
  }

  Iterable<Element> where(bool f(Element element)) {
    return IterableMixinWorkaround.where(this, f);
  }

  bool get isEmpty {
    return _element.$dom_firstElementChild == null;
  }

  List<Element> take(int n) {
    return IterableMixinWorkaround.takeList(this, n);
  }

  Iterable<Element> takeWhile(bool test(Element value)) {
    return IterableMixinWorkaround.takeWhile(this, test);
  }

  List<Element> skip(int n) {
    return IterableMixinWorkaround.skipList(this, n);
  }

  Iterable<Element> skipWhile(bool test(Element value)) {
    return IterableMixinWorkaround.skipWhile(this, test);
  }

  Element firstMatching(bool test(Element value), {Element orElse()}) {
    return IterableMixinWorkaround.firstMatching(this, test, orElse);
  }

  Element lastMatching(bool test(Element value), {Element orElse()}) {
    return IterableMixinWorkaround.lastMatchingInList(this, test, orElse);
  }

  Element singleMatching(bool test(Element value)) {
    return IterableMixinWorkaround.singleMatching(this, test);
  }

  Element elementAt(int index) {
    return this[index];
  }

  int get length {
    return _childElements.length;
  }

  Element operator [](int index) {
    return _childElements[index];
  }

  void operator []=(int index, Element value) {
    _element.$dom_replaceChild(value, _childElements[index]);
  }

  void set length(int newLength) {
    // TODO(jacobr): remove children when length is reduced.
    throw new UnsupportedError('');
  }

  Element add(Element value) {
    _element.$dom_appendChild(value);
    return value;
  }

  Element addLast(Element value) => add(value);

  Iterator<Element> get iterator => toList().iterator;

  void addAll(Iterable<Element> iterable) {
    for (Element element in iterable) {
      _element.$dom_appendChild(element);
    }
  }

  void sort([int compare(Element a, Element b)]) {
    throw new UnsupportedError('TODO(jacobr): should we impl?');
  }

  dynamic reduce(dynamic initialValue,
      dynamic combine(dynamic previousValue, Element element)) {
    return IterableMixinWorkaround.reduce(this, initialValue, combine);
  }

  void setRange(int start, int rangeLength, List from, [int startFrom = 0]) {
    throw new UnimplementedError();
  }

  void remove(Object object) {
    if (object is Element) {
      Element element = object;
      if (identical(element.parentNode, this)) {
        _element.$dom_removeChild(element);
      }
    }
  }

  void removeAll(Iterable elements) {
    Collections.removeAll(this, elements);
  }

  void retainAll(Iterable elements) {
    Collections.retainAll(this, elements);
  }

  void removeMatching(bool test(Element element)) {
    Collections.removeMatching(this, test);
  }

  void retainMatching(bool test(Element element)) {
    Collections.retainMatching(this, test);
  }

  void removeRange(int start, int rangeLength) {
    throw new UnimplementedError();
  }

  void insertRange(int start, int rangeLength, [initialValue = null]) {
    throw new UnimplementedError();
  }

  List getRange(int start, int rangeLength) =>
    new _FrozenElementList._wrap(Lists.getRange(this, start, rangeLength,
        []));

  int indexOf(Element element, [int start = 0]) {
    return Lists.indexOf(this, element, start, this.length);
  }

  int lastIndexOf(Element element, [int start = null]) {
    if (start == null) start = length - 1;
    return Lists.lastIndexOf(this, element, start);
  }

  void clear() {
    // It is unclear if we want to keep non element nodes?
    _element.text = '';
  }

  Element removeAt(int index) {
    final result = this[index];
    if (result != null) {
      _element.$dom_removeChild(result);
    }
    return result;
  }

  Element removeLast() {
    final result = this.last;
    if (result != null) {
      _element.$dom_removeChild(result);
    }
    return result;
  }

  Element get first {
    Element result = _element.$dom_firstElementChild;
    if (result == null) throw new StateError("No elements");
    return result;
  }


  Element get last {
    Element result = _element.$dom_lastElementChild;
    if (result == null) throw new StateError("No elements");
    return result;
  }

  Element get single {
    if (length > 1) throw new StateError("More than one element");
    return first;
  }

  Element min([int compare(Element a, Element b)]) {
    return IterableMixinWorkaround.min(this, compare);
  }

  Element max([int compare(Element a, Element b)]) {
    return IterableMixinWorkaround.max(this, compare);
  }
}

// TODO(jacobr): this is an inefficient implementation but it is hard to see
// a better option given that we cannot quite force NodeList to be an
// ElementList as there are valid cases where a NodeList JavaScript object
// contains Node objects that are not Elements.
class _FrozenElementList implements List {
  final List<Node> _nodeList;

  _FrozenElementList._wrap(this._nodeList);

  bool contains(Element element) {
    for (Element el in this) {
      if (el == element) return true;
    }
    return false;
  }

  void forEach(void f(Element element)) {
    for (Element el in this) {
      f(el);
    }
  }

  String join([String separator]) {
    return IterableMixinWorkaround.joinList(this, separator);
  }

  List mappedBy(f(Element element)) {
    return IterableMixinWorkaround.mappedByList(this, f);
  }

  Iterable<Element> where(bool f(Element element)) {
    return IterableMixinWorkaround.where(this, f);
  }

  bool every(bool f(Element element)) {
    for(Element element in this) {
      if (!f(element)) {
        return false;
      }
    };
    return true;
  }

  bool any(bool f(Element element)) {
    for(Element element in this) {
      if (f(element)) {
        return true;
      }
    };
    return false;
  }

  List<Element> toList() => new List<Element>.from(this);
  Set<Element> toSet() => new Set<Element>.from(this);

  List<Element> take(int n) {
    return IterableMixinWorkaround.takeList(this, n);
  }

  Iterable<Element> takeWhile(bool test(Element value)) {
    return IterableMixinWorkaround.takeWhile(this, test);
  }

  List<Element> skip(int n) {
    return IterableMixinWorkaround.skipList(this, n);
  }

  Iterable<Element> skipWhile(bool test(Element value)) {
    return IterableMixinWorkaround.skipWhile(this, test);
  }

  Element firstMatching(bool test(Element value), {Element orElse()}) {
    return IterableMixinWorkaround.firstMatching(this, test, orElse);
  }

  Element lastMatching(bool test(Element value), {Element orElse()}) {
    return IterableMixinWorkaround.lastMatchingInList(this, test, orElse);
  }

  Element singleMatching(bool test(Element value)) {
    return IterableMixinWorkaround.singleMatching(this, test);
  }

  Element elementAt(int index) {
    return this[index];
  }

  bool get isEmpty => _nodeList.isEmpty;

  int get length => _nodeList.length;

  Element operator [](int index) => _nodeList[index];

  void operator []=(int index, Element value) {
    throw new UnsupportedError('');
  }

  void set length(int newLength) {
    _nodeList.length = newLength;
  }

  void add(Element value) {
    throw new UnsupportedError('');
  }

  void addLast(Element value) {
    throw new UnsupportedError('');
  }

  Iterator<Element> get iterator => new _FrozenElementListIterator(this);

  void addAll(Iterable<Element> iterable) {
    throw new UnsupportedError('');
  }

  void sort([int compare(Element a, Element b)]) {
    throw new UnsupportedError('');
  }

  dynamic reduce(dynamic initialValue,
      dynamic combine(dynamic previousValue, Element element)) {
    return IterableMixinWorkaround.reduce(this, initialValue, combine);
  }

  void setRange(int start, int rangeLength, List from, [int startFrom = 0]) {
    throw new UnsupportedError('');
  }

  void removeRange(int start, int rangeLength) {
    throw new UnsupportedError('');
  }

  void insertRange(int start, int rangeLength, [initialValue = null]) {
    throw new UnsupportedError('');
  }

  List<Element> getRange(int start, int rangeLength) =>
    new _FrozenElementList._wrap(_nodeList.getRange(start, rangeLength));

  int indexOf(Element element, [int start = 0]) =>
    _nodeList.indexOf(element, start);

  int lastIndexOf(Element element, [int start = null]) =>
    _nodeList.lastIndexOf(element, start);

  void clear() {
    throw new UnsupportedError('');
  }

  Element removeAt(int index) {
    throw new UnsupportedError('');
  }

  Element removeLast() {
    throw new UnsupportedError('');
  }

  void remove(Object element) {
    throw new UnsupportedError('');
  }

  void removeAll(Iterable elements) {
    throw new UnsupportedError('');
  }

  void retainAll(Iterable elements) {
    throw new UnsupportedError('');
  }

  void removeMatching(bool test(Element element)) {
    throw new UnsupportedError('');
  }

  void retainMatching(bool test(Element element)) {
    throw new UnsupportedError('');
  }

  Element get first => _nodeList.first;

  Element get last => _nodeList.last;

  Element get single => _nodeList.single;

  Element min([int compare(Element a, Element b)]) {
    return IterableMixinWorkaround.min(this, compare);
  }

  Element max([int compare(Element a, Element b)]) {
    return IterableMixinWorkaround.max(this, compare);
  }
}

class _FrozenElementListIterator implements Iterator<Element> {
  final _FrozenElementList _list;
  int _index = -1;
  Element _current;

  _FrozenElementListIterator(this._list);

  /**
   * Moves to the next element. Returns true if the iterator is positioned
   * at an element. Returns false if it is positioned after the last element.
   */
  bool moveNext() {
    int nextIndex = _index + 1;
    if (nextIndex < _list.length) {
      _current = _list[nextIndex];
      _index = nextIndex;
      return true;
    }
    _index = _list.length;
    _current = null;
    return false;
  }

  /**
   * Returns the element the [Iterator] is positioned at.
   *
   * Return [:null:] if the iterator is positioned before the first, or
   * after the last element.
   */
  Element get current => _current;
}

class _ElementCssClassSet extends CssClassSet {

  final Element _element;

  _ElementCssClassSet(this._element);

  Set<String> readClasses() {
    var s = new Set<String>();
    var classname = _element.$dom_className;

    for (String name in classname.split(' ')) {
      String trimmed = name.trim();
      if (!trimmed.isEmpty) {
        s.add(trimmed);
      }
    }
    return s;
  }

  void writeClasses(Set<String> s) {
    List list = new List.from(s);
    _element.$dom_className = s.join(' ');
  }
}

/**
 * An abstract class, which all HTML elements extend.
 */
abstract class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {

  /**
   * Creates an HTML element from a valid fragment of HTML.
   *
   * The [html] fragment must represent valid HTML with a single element root,
   * which will be parsed and returned.
   *
   * Important: the contents of [html] should not contain any user-supplied
   * data. Without strict data validation it is impossible to prevent script
   * injection exploits.
   *
   * It is instead recommended that elements be constructed via [Element.tag]
   * and text be added via [text].
   *
   *     var element = new Element.html('<div class="foo">content</div>');
   */
  factory $CLASSNAME.html(String html) =>
      _$(CLASSNAME)FactoryProvider.createElement_html(html);

  /**
   * Creates the HTML element specified by the tag name.
   *
   * This is similar to [Document.createElement].
   * [tag] should be a valid HTML tag name. If [tag] is an unknown tag then
   * this will create an [UnknownElement].
   *
   *     var divElement = new Element.tag('div');
   *     print(divElement is DivElement); // 'true'
   *     var myElement = new Element.tag('unknownTag');
   *     print(myElement is UnknownElement); // 'true'
   *
   * For standard elements it is more preferable to use the type constructors:
   *     var element = new DivElement();
   *
   * See also:
   *
   * * [isTagSupported]
   */
  factory $CLASSNAME.tag(String tag) =>
      _$(CLASSNAME)FactoryProvider.createElement_tag(tag);

  /**
   * All attributes on this element.
   *
   * Any modifications to the attribute map will automatically be applied to
   * this element.
   *
   * This only includes attributes which are not in a namespace
   * (such as 'xlink:href'), additional attributes can be accessed via
   * [getNamespacedAttributes].
   */
  Map<String, String> get attributes => new _ElementAttributeMap(this);

  void set attributes(Map<String, String> value) {
    Map<String, String> attributes = this.attributes;
    attributes.clear();
    for (String key in value.keys) {
      attributes[key] = value[key];
    }
  }

  /**
   * Deprecated, use innerHtml instead.
   */
  @deprecated
  String get innerHTML => this.innerHtml;
  @deprecated
  void set innerHTML(String value) {
    this.innerHtml = value;
  }

  @deprecated
  void set elements(Collection<Element> value) {
    this.children = value;
  }

  /**
   * Deprecated, use [children] instead.
   */
  @deprecated
  List<Element> get elements => this.children;

  /**
   * List of the direct children of this element.
   *
   * This collection can be used to add and remove elements from the document.
   *
   *     var item = new DivElement();
   *     item.text = 'Something';
   *     document.body.children.add(item) // Item is now displayed on the page.
   *     for (var element in document.body.children) {
   *       element.style.background = 'red'; // Turns every child of body red.
   *     }
   */
  List<Element> get children => new _ChildrenElementList._wrap(this);

  void set children(List<Element> value) {
    // Copy list first since we don't want liveness during iteration.
    List copy = new List.from(value);
    var children = this.children;
    children.clear();
    children.addAll(copy);
  }

  /**
   * Finds the first descendant element of this element that matches the
   * specified group of selectors.
   *
   * [selectors] should be a string using CSS selector syntax.
   *
   *     // Gets the first descendant with the class 'classname'
   *     var element = element.query('.className');
   *     // Gets the element with id 'id'
   *     var element = element.query('#id');
   *     // Gets the first descendant [ImageElement]
   *     var img = element.query('img');
   *
   * See also:
   *
   * * [CSS Selectors](http://docs.webplatform.org/wiki/css/selectors)
   */
  Element query(String selectors) => $dom_querySelector(selectors);

  /**
   * Finds all descendent elements of this element that match the specified
   * group of selectors.
   *
   * [selectors] should be a string using CSS selector syntax.
   *
   *     var items = element.query('.itemClassName');
   */
  List<Element> queryAll(String selectors) =>
    new _FrozenElementList._wrap($dom_querySelectorAll(selectors));

  /**
   * The set of CSS classes applied to this element.
   *
   * This set makes it easy to add, remove or toggle the classes applied to
   * this element.
   *
   *     element.classes.add('selected');
   *     element.classes.toggle('isOnline');
   *     element.classes.remove('selected');
   */
  CssClassSet get classes => new _ElementCssClassSet(this);

  void set classes(Collection<String> value) {
    CssClassSet classSet = classes;
    classSet.clear();
    classSet.addAll(value);
  }

  /**
   * Allows access to all custom data attributes (data-*) set on this element.
   *
   * The keys for the map must follow these rules:
   *
   * * The name must not begin with 'xml'.
   * * The name cannot contain a semi-colon (';').
   * * The name cannot contain any capital letters.
   *
   * Any keys from markup will be converted to camel-cased keys in the map.
   *
   * For example, HTML specified as:
   *
   *     <div data-my-random-value='value'></div>
   *
   * Would be accessed in Dart as:
   *
   *     var value = element.dataAttributes['myRandomValue'];
   *
   * See also:
   *
   * * [Custom data attributes](http://www.w3.org/TR/html5/global-attributes.html#custom-data-attribute)
   */
  Map<String, String> get dataAttributes =>
    new _DataAttributeMap(attributes);

  void set dataAttributes(Map<String, String> value) {
    final dataAttributes = this.dataAttributes;
    dataAttributes.clear();
    for (String key in value.keys) {
      dataAttributes[key] = value[key];
    }
  }

  /**
   * Gets a map for manipulating the attributes of a particular namespace.
   *
   * This is primarily useful for SVG attributes such as xref:link.
   */
  Map<String, String> getNamespacedAttributes(String namespace) {
    return new _NamespacedAttributeMap(this, namespace);
  }

  /**
   * The set of all CSS values applied to this element, including inherited
   * and default values.
   *
   * The computedStyle contains values that are inherited from other
   * sources, such as parent elements or stylesheets. This differs from the
   * [style] property, which contains only the values specified directly on this
   * element.
   *
   * See also:
   *
   * * [CSS Inheritance and Cascade](http://docs.webplatform.org/wiki/tutorials/inheritance_and_cascade)
   */
  Future<CssStyleDeclaration> get computedStyle {
     // TODO(jacobr): last param should be null, see b/5045788
     return getComputedStyle('');
  }

  /**
   * Returns the computed styles for pseudo-elements such as `::after`,
   * `::before`, `::marker`, `::line-marker`.
   *
   * See also:
   *
   * * [Pseudo-elements](http://docs.webplatform.org/wiki/css/selectors/pseudo-elements)
   */
  Future<CssStyleDeclaration> getComputedStyle(String pseudoElement) {
    return _createMeasurementFuture(
        () => window.$dom_getComputedStyle(this, pseudoElement),
        new Completer<CssStyleDeclaration>());
  }

  /**
   * Adds the specified element to after the last child of this element.
   */
  void append(Element e) {
    this.children.add(e);
  }

  /**
   * Adds the specified text as a text node after the last child of this
   * element.
   */
  void appendText(String text) {
    this.insertAdjacentText('beforeend', text);
  }

  /**
   * Parses the specified text as HTML and adds the resulting node after the
   * last child of this element.
   */
  void appendHtml(String text) {
    this.insertAdjacentHtml('beforeend', text);
  }

  /**
   * Checks to see if the tag name is supported by the current platform.
   *
   * The tag should be a valid HTML tag name.
   */
  static bool isTagSupported(String tag) {
    var e = _ElementFactoryProvider.createElement_tag(tag);
    return e is Element && !(e is UnknownElement);
  }

  // Hooks to support custom WebComponents.
  /**
   * Experimental support for [web components][wc]. This field stores a
   * reference to the component implementation. It was inspired by Mozilla's
   * [x-tags][] project. Please note: in the future it may be possible to
   * `extend Element` from your class, in which case this field will be
   * deprecated and will simply return this [Element] object.
   *
   * [wc]: http://dvcs.w3.org/hg/webcomponents/raw-file/tip/explainer/index.html
   * [x-tags]: http://x-tags.org/
   */
$if DART2JS
  @Creates('Null')  // Set from Dart code; does not instantiate a native type.
$endif
  var xtag;

$if DART2JS
  static const EventStreamProvider<WheelEvent> mouseWheelEvent =
      const _CustomEventStreamProvider<WheelEvent>(
        Element._determineMouseWheelEventType);

  static String _determineMouseWheelEventType(EventTarget e) {
    if (JS('bool', '#.onwheel !== undefined', e)) {
      // W3C spec, and should be IE9+, but IE has a bug exposing onwheel.
      return 'wheel';
    } else if (JS('bool', '#.onmousewheel !== undefined', e)) {
      // Chrome & IE
      return 'mousewheel';
    } else {
      // Firefox
      return 'DOMMouseScroll';
    }
  }

  /**
   * Creates a text node and inserts it into the DOM at the specified location.
   *
   * To see the possible values for [where], read the doc for
   * [insertAdjacentHtml].
   *
   * See also:
   *
   * * [insertAdjacentHtml]
   */
  void insertAdjacentText(String where, String text) {
    if (JS('bool', '!!#.insertAdjacentText', this)) {
      _insertAdjacentText(where, text);
    } else {
      _insertAdjacentNode(where, new Text(text));
    }
  }

  @JSName('insertAdjacentText')
  void _insertAdjacentText(String where, String text) native;

  /**
   * Parses text as an HTML fragment and inserts it into the DOM at the
   * specified location.
   *
   * The [where] parameter indicates where to insert the HTML fragment:
   *
   * * 'beforeBegin': Immediately before this element.
   * * 'afterBegin': As the first child of this element.
   * * 'beforeEnd': As the last child of this element.
   * * 'afterEnd': Immediately after this element.
   *
   *     var html = '<div class="something">content</div>';
   *     // Inserts as the first child
   *     document.body.insertAdjacentHtml('afterBegin', html);
   *     var createdElement = document.body.children[0];
   *     print(createdElement.classes[0]); // Prints 'something'
   *
   * See also:
   *
   * * [insertAdjacentText]
   * * [insertAdjacentElement]
   */
  void insertAdjacentHtml(String where, String text) {
    if (JS('bool', '!!#.insertAdjacentHtml', this)) {
      _insertAdjacentHtml(where, text);
    } else {
      _insertAdjacentNode(where, new DocumentFragment.html(text));
    }
  }

  @JSName('insertAdjacentHTML')
  void _insertAdjacentHTML(String where, String text) native;

  /**
   * Inserts [element] into the DOM at the specified location.
   *
   * To see the possible values for [where], read the doc for
   * [insertAdjacentHtml].
   *
   * See also:
   *
   * * [insertAdjacentHtml]
   */
  Element insertAdjacentElement(String where, Element element) {
    if (JS('bool', '!!#.insertAdjacentElement', this)) {
      _insertAdjacentElement(where, element);
    } else {
      _insertAdjacentNode(where, element);
    }
    return element;
  }

  @JSName('insertAdjacentElement')
  void _insertAdjacentElement(String where, Element element) native;

  void _insertAdjacentNode(String where, Node node) {
    switch (where.toLowerCase()) {
      case 'beforebegin':
        this.parentNode.insertBefore(node, this);
        break;
      case 'afterbegin':
        var first = this.nodes.length > 0 ? this.nodes[0] : null;
        this.insertBefore(node, first);
        break;
      case 'beforeend':
        this.nodes.add(node);
        break;
      case 'afterend':
        this.parentNode.insertBefore(node, this.nextNode);
        break;
      default:
        throw new ArgumentError("Invalid position ${where}");
    }
  }

  /**
   * Checks if this element matches the CSS selectors.
   */
  @Experimental()
  bool matches(String selectors) {
    if (JS('bool', '!!#.matches', this)) {
      return JS('bool', '#.matches(#)', this, selectors);
    } else if (JS('bool', '!!#.webkitMatchesSelector', this)) {
      return JS('bool', '#.webkitMatchesSelector(#)', this, selectors);
    } else if (JS('bool', '!!#.mozMatchesSelector', this)) {
      return JS('bool', '#.mozMatchesSelector(#)', this, selectors);
    } else if (JS('bool', '!!#.msMatchesSelector', this)) {
      return JS('bool', '#.msMatchesSelector(#)', this, selectors);
    }
  }
$else
$endif

$!MEMBERS
}

final _START_TAG_REGEXP = new RegExp('<(\\w+)');
class _ElementFactoryProvider {
  static const _CUSTOM_PARENT_TAG_MAP = const {
    'body' : 'html',
    'head' : 'html',
    'caption' : 'table',
    'td': 'tr',
    'th': 'tr',
    'colgroup': 'table',
    'col' : 'colgroup',
    'tr' : 'tbody',
    'tbody' : 'table',
    'tfoot' : 'table',
    'thead' : 'table',
    'track' : 'audio',
  };

  // TODO(jmesserly): const set would be better
  static const _TABLE_TAGS = const {
    'caption': null,
    'col': null,
    'colgroup': null,
    'tbody': null,
    'td': null,
    'tfoot': null,
    'th': null,
    'thead': null,
    'tr': null,
  };

  @DomName('Document.createElement')
  static Element createElement_html(String html) {
    // TODO(jacobr): this method can be made more robust and performant.
    // 1) Cache the dummy parent elements required to use innerHTML rather than
    //    creating them every call.
    // 2) Verify that the html does not contain leading or trailing text nodes.
    // 3) Verify that the html does not contain both <head> and <body> tags.
    // 4) Detatch the created element from its dummy parent.
    String parentTag = 'div';
    String tag;
    final match = _START_TAG_REGEXP.firstMatch(html);
    if (match != null) {
      tag = match.group(1).toLowerCase();
      if (_Device.isIE && _TABLE_TAGS.containsKey(tag)) {
        return _createTableForIE(html, tag);
      }
      parentTag = _CUSTOM_PARENT_TAG_MAP[tag];
      if (parentTag == null) parentTag = 'div';
    }

    final temp = new Element.tag(parentTag);
    temp.innerHtml = html;

    Element element;
    if (temp.children.length == 1) {
      element = temp.children[0];
    } else if (parentTag == 'html' && temp.children.length == 2) {
      // In html5 the root <html> tag will always have a <body> and a <head>,
      // even though the inner html only contains one of them.
      element = temp.children[tag == 'head' ? 0 : 1];
    } else {
      _singleNode(temp.children);
    }
    element.remove();
    return element;
  }

  /**
   * IE table elements don't support innerHTML (even in standards mode).
   * Instead we use a div and inject the table element in the innerHtml string.
   * This technique works on other browsers too, but it's probably slower,
   * so we only use it when running on IE.
   *
   * See also innerHTML:
   * <http://msdn.microsoft.com/en-us/library/ie/ms533897(v=vs.85).aspx>
   * and Building Tables Dynamically:
   * <http://msdn.microsoft.com/en-us/library/ie/ms532998(v=vs.85).aspx>.
   */
  static Element _createTableForIE(String html, String tag) {
    var div = new Element.tag('div');
    div.innerHtml = '<table>$html</table>';
    var table = _singleNode(div.children);
    Element element;
    switch (tag) {
      case 'td':
      case 'th':
        element = _singleNode(_singleNode(table.rows).cells);
        break;
      case 'tr':
        element = _singleNode(table.rows);
        break;
      case 'tbody':
        element = _singleNode(table.tBodies);
        break;
      case 'thead':
        element = table.tHead;
        break;
      case 'tfoot':
        element = table.tFoot;
        break;
      case 'caption':
        element = table.caption;
        break;
      case 'colgroup':
        element = _getColgroup(table);
        break;
      case 'col':
        element = _singleNode(_getColgroup(table).children);
        break;
    }
    element.remove();
    return element;
  }

  static TableColElement _getColgroup(TableElement table) {
    // TODO(jmesserly): is there a better way to do this?
    return _singleNode(table.children.where((n) => n.tagName == 'COLGROUP')
        .toList());
  }

  static Node _singleNode(List<Node> list) {
    if (list.length == 1) return list[0];
    throw new ArgumentError('HTML had ${list.length} '
        'top level elements but 1 expected');
  }

  @DomName('Document.createElement')
$if DART2JS
  // Optimization to improve performance until the dart2js compiler inlines this
  // method.
  static dynamic createElement_tag(String tag) =>
      JS('Element', 'document.createElement(#)', tag);
$else
  static Element createElement_tag(String tag) =>
      document.$dom_createElement(tag);
$endif
}