Export the current Polymer version in polymer-element.js

[TimvdLippe]

Potential solution for exposing the Polymer version in polymer-element.js. Not sure if this is the desired solution, but it seemed like the best option we have.

Reference Issue

Fixes #5211

[kevinpschaaf]

Although this change makes introspecting the version from a running page possible, it is fairly cumbersome:

import('./need-to-know-path-to-polymer/polymer-element.js').then(module => console.log(module.version));

And that only works if a build tool hasn't compiled modules into bundles or out altogether.

I think it would be pretty useful to have e.g. a static getter on the ElementMixin return the version (in addition to a plain module export), so that it's more straightforward to introspect the version being used on a running page.

e.g. from an element in the inspector

console.log($0.constructor.polymerElementVersion);

or e.g. from the registry:

console.log(customElements.get('my-app').polymerElementVersion);

So perhaps the version can be exported from element-mixin.js instead of here, and returned from a new static get polymerElementVersion() { return version; } getter on the ElementMixin? It could also be re-exported from polymer-element.js for convenience.

[TimvdLippe]

@kevinpschaaf Good idea! Updated.

[TimvdLippe]

Note to myself: should update to element mixin

[TimvdLippe]

@kevinpschaaf This is ready for review again

[kevinpschaaf]

LGTM! Thanks