Changed

• Added a static getPropertyDescriptor method to allow easier customization of property accessors. This method should return a a PropertyDescriptor to install on the property. If no descriptor is returned, no property accessor is created. (#911)
• The value returned by render is always rendered, even if it isn't a TemplateResult. (#712)

Added

• Added @queryAsync(selector) decorator which returns a Promise that resolves to the result of querying for the given selector after the element's updateComplete Promise resolves (#903).
• Added enableUpdating() to UpdatingElement to enable customizing when updating is enabled #860.
• Added @queryAssignedNodes(slotName, flatten) decorator to enable querying assignedNodes for a given slot #860.
• Added getStyles() to LitElement to allow hooks into style gathering for component sets #866.

Fixed

• Ensure UpdatingElement allows updates when properties are set after calling super.update(). LitElement renders when updates are triggered as a result of rendering (#549).
• Properties annotated with the eventOptions decorator will now survive property renaming optimizations when used with tsickle and Closure JS Compiler.
• Moved style gathering from finalize to initialize to be more lazy, and create stylesheets on the first instance initializing #866.
• Fixed behavior change for components that do not implement render() introduced in (#712) (#917)