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diff --git a/toolkit/modules/Dict.jsm b/toolkit/modules/Dict.jsm
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["Dict"];
+
+/**
+ * Transforms a given key into a property name guaranteed not to collide with
+ * any built-ins.
+ */
+function convert(aKey) {
+  return ":" + aKey;
+}
+
+/**
+ * Transforms a property into a key suitable for providing to the outside world.
+ */
+function unconvert(aProp) {
+  return aProp.substr(1);
+}
+
+/**
+ * A dictionary of strings to arbitrary JS objects. This should be used whenever
+ * the keys are potentially arbitrary, to avoid collisions with built-in
+ * properties.
+ *
+ * @param aInitial An object containing the initial keys and values of this
+ *                 dictionary. Only the "own" enumerable properties of the
+ *                 object are considered.
+ *                 If |aInitial| is a string, it is assumed to be JSON and parsed into an object.
+ */
+this.Dict = function Dict(aInitial) {
+  if (aInitial === undefined)
+    aInitial = {};
+  if (typeof aInitial == "string")
+    aInitial = JSON.parse(aInitial);
+  var items = {}, count = 0;
+  // That we don't look up the prototype chain is guaranteed by Iterator.
+  for (var [key, val] in Iterator(aInitial)) {
+    items[convert(key)] = val;
+    count++;
+  }
+  this._state = {count: count, items: items};
+  return Object.freeze(this);
+}
+
+Dict.prototype = Object.freeze({
+  /**
+   * The number of items in the dictionary.
+   */
+  get count() {
+    return this._state.count;
+  },
+
+  /**
+   * Gets the value for a key from the dictionary. If the key is not a string,
+   * it will be converted to a string before the lookup happens.
+   *
+   * @param aKey The key to look up
+   * @param [aDefault] An optional default value to return if the key is not
+   *                   present. Defaults to |undefined|.
+   * @returns The item, or aDefault if it isn't found.
+   */
+  get: function Dict_get(aKey, aDefault) {
+    var prop = convert(aKey);
+    var items = this._state.items;
+    return items.hasOwnProperty(prop) ? items[prop] : aDefault;
+  },
+
+  /**
+   * Sets the value for a key in the dictionary. If the key is a not a string,
+   * it will be converted to a string before the set happens.
+   */
+  set: function Dict_set(aKey, aValue) {
+    var prop = convert(aKey);
+    var items = this._state.items;
+    if (!items.hasOwnProperty(prop))
+      this._state.count++;
+    items[prop] = aValue;
+  },
+
+  /**
+   * Sets a lazy getter function for a key's value. If the key is a not a string,
+   * it will be converted to a string before the set happens.
+   * @param aKey
+   *        The key to set
+   * @param aThunk
+   *        A getter function to be called the first time the value for aKey is
+   *        retrieved. It is guaranteed that aThunk wouldn't be called more
+   *        than once.  Note that the key value may be retrieved either
+   *        directly, by |get|, or indirectly, by |listvalues| or by iterating
+   *        |values|.  For the later, the value is only retrieved if and when
+   *        the iterator gets to the value in question.  Also note that calling
+   *        |has| for a lazy-key does not invoke aThunk.
+   *
+   * @note No context is provided for aThunk when it's invoked.
+   *       Use Function.bind if you wish to run it in a certain context.
+   */
+  setAsLazyGetter: function Dict_setAsLazyGetter(aKey, aThunk) {
+    let prop = convert(aKey);
+    let items = this._state.items;
+    if (!items.hasOwnProperty(prop))
+      this._state.count++;
+
+    Object.defineProperty(items, prop, {
+      get: function() {
+        delete items[prop];
+        return items[prop] = aThunk();
+      },
+      configurable: true,
+      enumerable: true     
+    });
+  },
+
+  /**
+   * Returns whether a key is set as a lazy getter.  This returns
+   * true only if the getter function was not called already.
+   * @param aKey
+   *        The key to look up.
+   * @returns whether aKey is set as a lazy getter.
+   */
+  isLazyGetter: function Dict_isLazyGetter(aKey) {
+    let descriptor = Object.getOwnPropertyDescriptor(this._state.items,
+                                                     convert(aKey));
+    return (descriptor && descriptor.get != null);
+  },
+
+  /**
+   * Returns whether a key is in the dictionary. If the key is a not a string,
+   * it will be converted to a string before the lookup happens.
+   */
+  has: function Dict_has(aKey) {
+    return (this._state.items.hasOwnProperty(convert(aKey)));
+  },
+
+  /**
+   * Deletes a key from the dictionary. If the key is a not a string, it will be
+   * converted to a string before the delete happens.
+   *
+   * @returns true if the key was found, false if it wasn't.
+   */
+  del: function Dict_del(aKey) {
+    var prop = convert(aKey);
+    if (this._state.items.hasOwnProperty(prop)) {
+      delete this._state.items[prop];
+      this._state.count--;
+      return true;
+    }
+    return false;
+  },
+
+  /**
+   * Returns a shallow copy of this dictionary.
+   */
+  copy: function Dict_copy() {
+    var newItems = {};
+    for (var [key, val] in this.items)
+      newItems[key] = val;
+    return new Dict(newItems);
+  },
+
+  /*
+   * List and iterator functions
+   *
+   * No guarantees whatsoever are made about the order of elements.
+   */
+
+  /**
+   * Returns a list of all the keys in the dictionary in an arbitrary order.
+   */
+  listkeys: function Dict_listkeys() {
+    // Tycho: return [unconvert(k) for (k in this._state.items)];
+    let list = [];
+    for (let k in this._state.items) {
+      list.push(unconvert(k));
+    }
+    return list;
+  },
+
+  /**
+   * Returns a list of all the values in the dictionary in an arbitrary order.
+   */
+  listvalues: function Dict_listvalues() {
+    let items = this._state.items;
+    // Tycho: return [items[k] for (k in items)];
+    let list = [];
+    for (let k in items) {
+      list.push(items[k]);
+    }
+    return list;
+  },
+
+  /**
+   * Returns a list of all the items in the dictionary as key-value pairs
+   * in an arbitrary order.
+   */
+  listitems: function Dict_listitems() {
+    let items = this._state.items;
+    // Tycho: return [[unconvert(k), items[k]] for (k in items)];
+    let list = [];
+    let obj;
+    for (let k in items) {
+      obj = {};
+      obj[unconvert(k)] = items[k];
+      list.push(obj);
+    }
+    return list;
+  },
+
+  /**
+   * Returns an iterator over all the keys in the dictionary in an arbitrary
+   * order. No guarantees are made about what happens if the dictionary is
+   * mutated during iteration.
+   */
+  get keys() {
+    // If we don't capture this._state.items here then the this-binding will be
+    // incorrect when the generator is executed
+    let items = this._state.items;
+    // Tycho: return (unconvert(k) for (k in items));
+    let list = [];
+    for (let k in items) {
+      list.push(unconvert(k));
+    }
+    return list;
+  },
+
+  /**
+   * Returns an iterator over all the values in the dictionary in an arbitrary
+   * order. No guarantees are made about what happens if the dictionary is
+   * mutated during iteration.
+   */
+  get values() {
+    // If we don't capture this._state.items here then the this-binding will be
+    // incorrect when the generator is executed
+    let items = this._state.items;
+    // Tycho: return (items[k] for (k in items));
+    let list = [];
+    for (let k in items) {
+      list.push(items[k]);
+    }
+    return list;
+  },
+
+  /**
+   * Returns an iterator over all the items in the dictionary as key-value pairs
+   * in an arbitrary order. No guarantees are made about what happens if the
+   * dictionary is mutated during iteration.
+   */
+  get items() {
+    // If we don't capture this._state.items here then the this-binding will be
+    // incorrect when the generator is executed
+    let items = this._state.items;
+    // Tycho: return ([unconvert(k), items[k]] for (k in items));
+    let list = [];
+    let obj;
+    for each (let k in items) {
+      obj = {};
+      obj[unconvert(k)] = items[k];
+      list.push(obj);
+    }
+    return list;
+  },
+
+  /**
+   * Returns a String representation of this dictionary.
+   */
+  toString: function Dict_toString() {
+    /* Tycho:
+    return "{" +
+      [(key + ": " + val) for ([key, val] in this.items)].join(", ") +
+      "}";
+    */
+    let list = this.items.map(function (item) {
+      return item["key"] + ": " + item["val"];
+    });
+    return "{" + list.join(", ") + "}";
+  },
+
+  /**
+   * Returns a JSON representation of this dictionary.
+   */
+  toJSON: function Dict_toJSON() {
+    let obj = {};
+    for (let [key, item] of Iterator(this._state.items)) {
+      obj[unconvert(key)] = item;
+    }
+    return JSON.stringify(obj);
+  },
+});