Poylmerize - Polymer 2.0 Dart experimental support

This package is a command line tool to build Polymer 2 components with DDC from Dart.

The benefits of this approach compared to the dart2js standard polymer-1.x are :

  • support for polymer 2.0-preview (web components 1.0)
  • using DDC to generate ES6 output code
  • dynamic load of polymer components definitions through requirejs
  • interoperability with other JS frameworks
  • Incremental build (dependencies are built only once)
  • possibility to distribute ONLY the build result to thirdy party users and devs
  • simplified API
  • automatic getter and setter (no explicit notify for first level properties)
  • NO Annotations required to expose properties
  • NO Annotations required to expose methods


Too good to be true ? Well the bad news is that although very promising this package is based on the EXPERIMENTAL DEV COMPILER and therefore this is to be considered HIGHLY UNSTABLE and not ready for production.

Nevertheless it can be though as a POC to demostrate the extremely high potential of this approach for Dart.

This tool is tested ONLY on Linux. Should work on other unix based system. Probably will not work on windows.

See NOTES.md for details.


Install with pub global activate polymerize.


A sample project demostrating how to build polymer-2 components using polymerize can be found here : - Sample mini todo APP for Polymer2-Dart-DDC Project

See the README for more information.

Launch the build with the following command in the main package dir:

  • polymerize

(use polymerize -h for more options).

Component definition

This is a sample component definition:

import 'package:polymer_element/polymer_element.dart'

class MyTag extends PolymerElement {

  int count = 0;  // <- no need to annotate this !!!

  onClickIt(Event ev,details) {  // <- NO need to annotate this!!!!
    count = count + 1;    // <- no need to call `set` API , magical setter in action here

  void countChanged(val) {
    print("Count has changed : ${count}");

  MyTag() { // <- Use a simple constructor for created callback !!!
    print("HELLO THERE !")

  factory MyTag.tag() => Element.tag('my-tag'); // <- If you want to create it programmatically use this

  connectedCallback() {
    super.connectedCallback(); // <- MUST BE CALLED !!!!

The Html template is just the usual dom-module template without any JS code and with <link> to import other polymer dependencies (like polymer2 itself and any used component). The index.html should preload requirejs, webcomponents polyfill and polymer.html (see the demo).


The build tool will operate in this way :

  • Every dependency of the main package will be processed and will produce a separate loadable module in the output Directory
  • For every polymer component a new html file will be produced that will load the original template and will load the corresponding dart class

Every file with ".dart" extension inside the lib folder of a dependency will be considered in the build of the corresponding module.

Every other file in the lib folder will be considered an asset and compied to the final build destination folder.

No other folder will be considered in the build. The only exception is the web folder in the main package that will be copied to the final build destination folder.

Compilation for hosted packages will be cached inside the folder $HOME/.polymerize (inside the current directory) and reused without rebuilding it for the next build.


  • more polymer APIs
  • better convert to/from JS
  • annotations for properties (computed props, etc.)
  • support for mixins
  • support for external element wrappers
  • support for auto gen HTML imports