Version 1.49.90

- New API: GObject.registerClass(), intended for use with ES6 classes. When
  defining a GObject class using ES6 syntax, you must call
  GObject.registerClass() on the class object, with an optional metadata
  object as the first argument. (The metadata object works exactly like the
  meta properties from Lang.Class, except that Name and Extends are not
  present.)

  Old:

      var MyClass = new Lang.Class({
          Name: 'MyClass',
          Extends: GObject.Object,
          Signals: { 'event': {} },
          _init(props={}) {
              this._private = [];
              this.parent(props);
          },
      });

  New:

      var MyClass = GObject.registerClass({
          Signals: { 'event': {} },
      }, class MyClass extends GObject.Object {
          _init(props={}) {
              this._private = [];
              super._init(props);
          }
      });

  It is forward compatible with the following syntax requiring decorators and
  class fields, which are not in the JS standard yet:

      @GObject.registerClass
      class MyClass extends GObject.Object {
          static [GObject.signals] = { 'event': {} }
          _init(props={}) {
              this._private = [];
              super._init(props);
          }
      }

  One limitation is that GObject ES6 classes can't have constructor()
  methods, they must do any setup in an _init() method. This may be able to be
  fixed in the future.

- Closed bugs:

  * Misc 1.49 and mozjs52 enhancements [#785040, Philip Chimento]
  * Switch to native promises [#784713, Philip Chimento]
  * Can't call exports using top-level variable toString [#781623, Philip
    Chimento]
  * Properties no longer recognized when shadowed by a method [#785091, Philip
    Chimento, Rico Tzschichholz]
  * Patch: backport of changes required for use with mozjs-55 [#785424, Luke
    Jones]
This tag has no release notes.