text: let MutationObserver = window.MutationObserver
    || window.WebKitMutationObserver
    || window.MozMutationObserver;
  
  /*
   * Copyright 2012 The Polymer Authors. All rights reserved.
   * Use of this source code is goverened by a BSD-style
   * license that can be found in the LICENSE file.
   */
  
  let WeakMap = window.WeakMap;
  
  if (typeof WeakMap === 'undefined') {
    const defineProperty = Object.defineProperty;
    let counter = Date.now() % 1e9;
  
    WeakMap = function() {
      this.name = '__st' + (Math.random() * 1e9 >>> 0) + (counter++ + '__');
    };
  
    WeakMap.prototype = {
      set: function(key, value) {
        const entry = key[this.name];
        if (entry && entry[0] === key)
          entry[1] = value;
        else
          defineProperty(key, this.name, {value: [key, value], writable: true});
        return this;
      },
      get: function(key) {
        let entry;
        return (entry = key[this.name]) && entry[0] === key ?
            entry[1] : undefined;
      },
      'delete': function(key) {
        const entry = key[this.name];
        if (!entry) return false;
        const hasValue = entry[0] === key;
        entry[0] = entry[1] = undefined;
        return hasValue;
      },
      has: function(key) {
        const entry = key[this.name];
        if (!entry) return false;
        return entry[0] === key;
      }
    };
  }
  
  const registrationsTable = new WeakMap();
  
  // We use setImmediate or postMessage for our future callback.
  let setImmediate = window.msSetImmediate;
  
  // Use post message to emulate setImmediate.
  if (!setImmediate) {
    let setImmediateQueue = [];
    const sentinel = String(Math.random());
    window.addEventListener('message', function(e) {
      if (e.data === sentinel) {
        const queue = setImmediateQueue;
        setImmediateQueue = [];
        queue.forEach(function(func) {
          func();
        });
      }
    });
    setImmediate = function(func) {
      setImmediateQueue.push(func);
      window.postMessage(sentinel, '*');
    };
  }
  
  // This is used to ensure that we never schedule 2 callas to setImmediate
  let isScheduled = false;
  
  // Keep track of observers that needs to be notified next time.
  let scheduledObservers = [];
  
  /**
   * Schedules |dispatchCallback| to be called in the future.
   * @param {MutationObserver} observer
   */
  function scheduleCallback(observer) {
    scheduledObservers.push(observer);
    if (!isScheduled) {
      isScheduled = true;
      setImmediate(dispatchCallbacks);
    }
  }
  
  function wrapIfNeeded(node) {
    return window.ShadowDOMPolyfill &&
        window.ShadowDOMPolyfill.wrapIfNeeded(node) ||
        node;
  }
  
  function dispatchCallbacks() {
    // http://dom.spec.whatwg.org/#mutation-observers
  
    isScheduled = false; // Used to allow a new setImmediate call above.
  
    const observers = scheduledObservers;
    scheduledObservers = [];
    // Sort observers based on their creation UID (incremental).
    observers.sort(function(o1, o2) {
      return o1.uid_ - o2.uid_;
    });
  
    let anyNonEmpty = false;
    observers.forEach(function(observer) {
  
      // 2.1, 2.2
      const queue = observer.takeRecords();
      // 2.3. Remove all transient registered observers whose observer is mo.
      removeTransientObserversFor(observer);
  
      // 2.4
      if (queue.length) {
        observer.callback_(queue, observer);
        anyNonEmpty = true;
      }
    });
  
    // 3.
    if (anyNonEmpty)
      dispatchCallbacks();
  }
  
  function removeTransientObserversFor(observer) {
    observer.nodes_.forEach(function(node) {
      const registrations = registrationsTable.get(node);
      if (!registrations)
        return;
      registrations.forEach(function(registration) {
        if (registration.observer === observer)
          registration.removeTransientObservers();
      });
    });
  }
  
  /**
   * This function is used for the "For each registered observer observer (with
   * observer's options as options) in target's list of registered observers,
   * run these substeps:" and the "For each ancestor ancestor of target, and for
   * each registered observer observer (with options options) in ancestor's list
   * of registered observers, run these substeps:" part of the algorithms. The
   * |options.subtree| is checked to ensure that the callback is called
   * correctly.
   *
   * @param {Node} target
   * @param {function(MutationObserverInit):MutationRecord} callback
   */
  function forEachAncestorAndObserverEnqueueRecord(target, callback) {
    for (let node = target; node; node = node.parentNode) {
      const registrations = registrationsTable.get(node);
  
      if (registrations) {
        for (let j = 0; j < registrations.length; j++) {
          const registration = registrations[j];
          const options = registration.options;
  
          // Only target ignores subtree.
          if (node !== target && !options.subtree)
            continue;
  
          const record = callback(options);
          if (record)
            registration.enqueue(record);
        }
      }
    }
  }
  
  let uidCounter = 0;
  
  /**
   * The class that maps to the DOM MutationObserver interface.
   * @param {Function} callback.
   * @constructor
   */
  function JsMutationObserver(callback) {
    this.callback_ = callback;
    this.nodes_ = [];
    this.records_ = [];
    this.uid_ = ++uidCounter;
  }
  
  JsMutationObserver.prototype = {
    observe: function(target, options) {
      target = wrapIfNeeded(target);
  
      // 1.1
      if (!options.childList && !options.attributes && !options.characterData ||
  
          // 1.2
          options.attributeOldValue && !options.attributes ||
  
          // 1.3
          options.attributeFilter && options.attributeFilter.length &&
              !options.attributes ||
  
          // 1.4
          options.characterDataOldValue && !options.characterData) {
  
        throw new SyntaxError();
      }
  
      let registrations = registrationsTable.get(target);
      if (!registrations)
        registrationsTable.set(target, registrations = []);
  
      // 2
      // If target's list of registered observers already includes a registered
      // observer associated with the context object, replace that registered
      // observer's options with options.
      let registration;
      for (let i = 0; i < registrations.length; i++) {
        if (registrations[i].observer === this) {
          registration = registrations[i];
          registration.removeListeners();
          registration.options = options;
          break;
        }
      }
  
      // 3.
      // Otherwise, add a new registered observer to target's list of registered
      // observers with the context object as the observer and options as the
      // options, and add target to context object's list of nodes on which it
      // is registered.
      if (!registration) {
        registration = new Registration(this, target, options);
        registrations.push(registration);
        this.nodes_.push(target);
      }
  
      registration.addListeners();
    },
  
    disconnect: function() {
      this.nodes_.forEach(function(node) {
        const registrations = registrationsTable.get(node);
        for (let i = 0; i < registrations.length; i++) {
          const registration = registrations[i];
          if (registration.observer === this) {
            registration.removeListeners();
            registrations.splice(i, 1);
            // Each node can only have one registered observer associated with
            // this observer.
            break;
          }
        }
      }, this);
      this.records_ = [];
    },
  
    takeRecords: function() {
      const copyOfRecords = this.records_;
      this.records_ = [];
      return copyOfRecords;
    }
  };
  
  /**
   * @param {string} type
   * @param {Node} target
   * @constructor
   */
  function MutationRecord(type, target) {
    this.type = type;
    this.target = target;
    this.addedNodes = [];
    this.removedNodes = [];
    this.previousSibling = null;
    this.nextSibling = null;
    this.attributeName = null;
    this.attributeNamespace = null;
    this.oldValue = null;
  }
  
  function copyMutationRecord(original) {
    const record = new MutationRecord(original.type, original.target);
    record.addedNodes = original.addedNodes.slice();
    record.removedNodes = original.removedNodes.slice();
    record.previousSibling = original.previousSibling;
    record.nextSibling = original.nextSibling;
    record.attributeName = original.attributeName;
    record.attributeNamespace = original.attributeNamespace;
    record.oldValue = original.oldValue;
    return record;
  }
  
  // We keep track of the two (possibly one) records used in a single mutation.
  let currentRecord, recordWithOldValue;
  
  /**
   * Creates a record without |oldValue| and caches it as |currentRecord| for
   * later use.
   * @param {string} oldValue
   * @return {MutationRecord}
   */
  function getRecord(type, target) {
    return currentRecord = new MutationRecord(type, target);
  }
  
  /**
   * Gets or creates a record with |oldValue| based in the |currentRecord|
   * @param {string} oldValue
   * @return {MutationRecord}
   */
  function getRecordWithOldValue(oldValue) {
    if (recordWithOldValue)
      return recordWithOldValue;
    recordWithOldValue = copyMutationRecord(currentRecord);
    recordWithOldValue.oldValue = oldValue;
    return recordWithOldValue;
  }
  
  function clearRecords() {
    currentRecord = recordWithOldValue = undefined;
  }
  
  /**
   * @param {MutationRecord} record
   * @return {boolean} Whether the record represents a record from the current
   * mutation event.
   */
  function recordRepresentsCurrentMutation(record) {
    return record === recordWithOldValue || record === currentRecord;
  }
  
  /**
   * Selects which record, if any, to replace the last record in the queue.
   * This returns |null| if no record should be replaced.
   *
   * @param {MutationRecord} lastRecord
   * @param {MutationRecord} newRecord
   * @param {MutationRecord}
   */
  function selectRecord(lastRecord, newRecord) {
    if (lastRecord === newRecord)
      return lastRecord;
  
    // Check if the the record we are adding represents the same record. If
    // so, we keep the one with the oldValue in it.
    if (recordWithOldValue && recordRepresentsCurrentMutation(lastRecord))
      return recordWithOldValue;
  
    return null;
  }
  
  /**
   * Class used to represent a registered observer.
   * @param {MutationObserver} observer
   * @param {Node} target
   * @param {MutationObserverInit} options
   * @constructor
   */
  function Registration(observer, target, options) {
    this.observer = observer;
    this.target = target;
    this.options = options;
    this.transientObservedNodes = [];
  }
  
  Registration.prototype = {
    enqueue: function(record) {
      const records = this.observer.records_;
      const length = records.length;
  
      // There are cases where we replace the last record with the new record.
      // For example if the record represents the same mutation we need to use
      // the one with the oldValue. If we get same record (this can happen as we
      // walk up the tree) we ignore the new record.
      if (records.length > 0) {
        const lastRecord = records[length - 1];
        const recordToReplaceLast = selectRecord(lastRecord, record);
        if (recordToReplaceLast) {
          records[length - 1] = recordToReplaceLast;
          return;
        }
      } else {
        scheduleCallback(this.observer);
      }
  
      records[length] = record;
    },
  
    addListeners: function() {
      this.addListeners_(this.target);
    },
  
    addListeners_: function(node) {
      const options = this.options;
      if (options.attributes)
        node.addEventListener('DOMAttrModified', this, true);
  
      if (options.characterData)
        node.addEventListener('DOMCharacterDataModified', this, true);
  
      if (options.childList)
        node.addEventListener('DOMNodeInserted', this, true);
  
      if (options.childList || options.subtree)
        node.addEventListener('DOMNodeRemoved', this, true);
    },
  
    removeListeners: function() {
      this.removeListeners_(this.target);
    },
  
    removeListeners_: function(node) {
      const options = this.options;
      if (options.attributes)
        node.removeEventListener('DOMAttrModified', this, true);
  
      if (options.characterData)
        node.removeEventListener('DOMCharacterDataModified', this, true);
  
      if (options.childList)
        node.removeEventListener('DOMNodeInserted', this, true);
  
      if (options.childList || options.subtree)
        node.removeEventListener('DOMNodeRemoved', this, true);
    },
  
    /**
     * Adds a transient observer on node. The transient observer gets removed
     * next time we deliver the change records.
     * @param {Node} node
     */
    addTransientObserver: function(node) {
      // Don't add transient observers on the target itself. We already have all
      // the required listeners set up on the target.
      if (node === this.target)
        return;
  
      this.addListeners_(node);
      this.transientObservedNodes.push(node);
      let registrations = registrationsTable.get(node);
      if (!registrations)
        registrationsTable.set(node, registrations = []);
  
      // We know that registrations does not contain this because we already
      // checked if node === this.target.
      registrations.push(this);
    },
  
    removeTransientObservers: function() {
      const transientObservedNodes = this.transientObservedNodes;
      this.transientObservedNodes = [];
  
      transientObservedNodes.forEach(function(node) {
        // Transient observers are never added to the target.
        this.removeListeners_(node);
  
        const registrations = registrationsTable.get(node);
        for (let i = 0; i < registrations.length; i++) {
          if (registrations[i] === this) {
            registrations.splice(i, 1);
            // Each node can only have one registered observer associated with
            // this observer.
            break;
          }
        }
      }, this);
    },
  
    handleEvent: function(e) {
      // Stop propagation since we are managing the propagation manually.
      // This means that other mutation events on the page will not work
      // correctly but that is by design.
      e.stopImmediatePropagation();
  
      switch (e.type) {
        case 'DOMAttrModified':
          // http://dom.spec.whatwg.org/#concept-mo-queue-attributes
  
          var name = e.attrName;
          var namespace = e.relatedNode.namespaceURI;
          var target = e.target;
  
          // 1.
          var record = new getRecord('attributes', target);
          record.attributeName = name;
          record.attributeNamespace = namespace;
  
          // 2.
          var oldValue = null;
          if (!(typeof MutationEvent !== 'undefined' && e.attrChange === MutationEvent.ADDITION))
            oldValue = e.prevValue;
  
          forEachAncestorAndObserverEnqueueRecord(target, function(options) {
            // 3.1, 4.2
            if (!options.attributes)
              return;
  
            // 3.2, 4.3
            if (options.attributeFilter && options.attributeFilter.length &&
                options.attributeFilter.indexOf(name) === -1 &&
                options.attributeFilter.indexOf(namespace) === -1) {
              return;
            }
            // 3.3, 4.4
            if (options.attributeOldValue)
              return getRecordWithOldValue(oldValue);
  
            // 3.4, 4.5
            return record;
          });
  
          break;
  
        case 'DOMCharacterDataModified':
          // http://dom.spec.whatwg.org/#concept-mo-queue-characterdata
          var target = e.target;
  
          // 1.
          var record = getRecord('characterData', target);
  
          // 2.
          var oldValue = e.prevValue;
  
  
          forEachAncestorAndObserverEnqueueRecord(target, function(options) {
            // 3.1, 4.2
            if (!options.characterData)
              return;
  
            // 3.2, 4.3
            if (options.characterDataOldValue)
              return getRecordWithOldValue(oldValue);
  
            // 3.3, 4.4
            return record;
          });
  
          break;
  
        case 'DOMNodeRemoved':
          this.addTransientObserver(e.target);
          // Fall through.
        case 'DOMNodeInserted':
          // http://dom.spec.whatwg.org/#concept-mo-queue-childlist
          var target = e.relatedNode;
          var changedNode = e.target;
          var addedNodes, removedNodes;
          if (e.type === 'DOMNodeInserted') {
            addedNodes = [changedNode];
            removedNodes = [];
          } else {
  
            addedNodes = [];
            removedNodes = [changedNode];
          }
          var previousSibling = changedNode.previousSibling;
          var nextSibling = changedNode.nextSibling;
  
          // 1.
          var record = getRecord('childList', target);
          record.addedNodes = addedNodes;
          record.removedNodes = removedNodes;
          record.previousSibling = previousSibling;
          record.nextSibling = nextSibling;
  
          forEachAncestorAndObserverEnqueueRecord(target, function(options) {
            // 2.1, 3.2
            if (!options.childList)
              return;
  
            // 2.2, 3.3
            return record;
          });
  
      }
  
      clearRecords();
    }
  };
  
  if (!MutationObserver) {
    MutationObserver = JsMutationObserver;
  }
  
  module.exports = MutationObserver;
  